RGB Color mode
Photoshop RGB Color mode uses the RGB model, assigning an intensity value to each pixel. In 8‑bits-per-channel images, the intensity values range from 0 (black) to 255 (white) for each of the RGB (red, green, blue) components in a color image. For example, a bright red color has an R value of 246, a G value of 20, and a B value of 50. When the values of all three components are equal, the result is a shade of neutral gray. When the values of all components are 255, the result is pure white; when the values are 0, pure black.
RGB images use three colors, or channels, to reproduce colors on screen. In 8‑bits-per-channel images, the three channels translate to 24 (8 bits x 3 channels) bits of color information per pixel. With 24‑bit images, the three channels can reproduce up to 16.7 million colors per pixel. With 48‑bit (16‑bits-per-channel) and 96‑bit (32‑bits-per-channel) images, even more colors can be reproduced per pixel. In addition to being the default mode for new Photoshop images, the RGB model is used by computer monitors to display colors. This means that when working in color modes other than RGB, such as CMYK, Photoshop converts the CMYK image to RGB for display on screen.
Although RGB is a standard color model, the exact range of colors represented can vary, depending on the application or display device. The RGB Color mode in Photoshop varies according to the working space setting that you specify in the Color Settings dialog box.
CMYK Color mode
In the CMYK mode, each pixel is assigned a percentage value for each of the process inks. The lightest (highlight) colors are assigned small percentages of process ink colors; the darker (shadow) colors higher percentages. For example, a bright red might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In CMYK images, pure white is generated when all four components have values of 0%.
Use the CMYK mode when preparing an image to be printed using process colors. Converting an RGB image into CMYK creates a color separation. If you start with an RGB image, it’s best to edit first in RGB and then convert to CMYK at the end of your editing process. In RGB mode, you can use the Proof Setup commands to simulate the effects of a CMYK conversion without changing the actual image data. You can also use CMYK mode to work directly with CMYK images scanned or imported from high-end systems.
Although CMYK is a standard color model, the exact range of colors represented can vary, depending on the press and printing conditions. The CMYK Color mode in Photoshop varies according to the working space setting that you specify in the Color Settings dialog box.
Lab Color mode
The CIE L*a*b* color model (Lab) is based on the human perception of color. The numeric values in Lab describe all the colors that a person with normal vision sees. Because Lab describes how a color looks rather than how much of a particular colorant is needed for a device (such as a monitor, desktop printer, or digital camera) to produce colors, Lab is considered to be a device-independentcolor model. Color management systems use Lab as a color reference to predictably transform a color from one color space to another color space.
The Lab Color mode has a lightness component (L) that can range from 0 to 100. In the Adobe Color Picker and Color panel, the a component (green-red axis) and the b component (blue-yellow axis) can range from +127 to –128.
Lab images can be saved in Photoshop, Photoshop EPS, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, TIFF, Photoshop DCS 1.0, or Photoshop DCS 2.0 formats. You can save 48‑bit (16‑bits-per-channel) Lab images in Photoshop, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, or TIFF formats.
Note: The DCS 1.0 and DCS 2.0 formats convert the file to CMYK when opened.
Grayscale mode
Grayscale mode uses different shades of gray in an image. In 8‑bit images, there can be up to 256 shades of gray. Every pixel of a grayscale image has a brightness value ranging from 0 (black) to 255 (white). In 16 and 32‑bit images, the number of shades in an image is much greater than in 8‑bit images.
Grayscale values can also be measured as percentages of black ink coverage (0% is equal to white, 100% to black).
Grayscale mode uses the range defined by the working space setting that you specify in the Color Settings dialog box.
Bitmap mode
Bitmap mode uses one of two color values (black or white) to represent the pixels in an image. Images in Bitmap mode are called bitmapped 1‑bit images because they have a bit depth of 1.
Duotone mode
Duotone mode creates monotone, duotone (two-color), tritone (three-color), and quadtone (four-color) grayscale images using one to four custom inks.
Indexed Color mode
Indexed Color mode produces 8‑bit image files with up to 256 colors. When converting to indexed color, Photoshop builds a color lookup table (CLUT), which stores and indexes the colors in the image. If a color in the original image does not appear in the table, the program chooses the closest one or uses dithering to simulate the color using available colors.
Although its palette of colors is limited, indexed color can reduce file size yet maintain the visual quality needed for multimedia presentations, web pages, and the like. Limited editing is available in this mode. For extensive editing, you should convert temporarily to RGB mode. Indexed color files can be saved in Photoshop, BMP, DICOM (Digital Imaging and Communications in Medicine), GIF, Photoshop EPS, Large Document Format (PSB), PCX, Photoshop PDF, Photoshop Raw, Photoshop 2.0, PICT, PNG, Targa®, or TIFF formats.
Multichannel mode
Multichannel mode images contain 256 levels of gray in each channel and are useful for specialized printing. Multichannel mode images can be saved in Photoshop, Large Document Format (PSB), Photoshop 2.0, Photoshop Raw, or Photoshop DCS 2.0 formats.
These guidelines apply when converting images to Multichannel mode:
- Layers are unsupported and therefore flattened.
- Color channels in the original image become spot color channels in the converted image.
- Converting a CMYK image to Multichannel mode creates cyan, magenta, yellow, and black spot channels.
- Converting an RGB image to Multichannel mode creates cyan, magenta, and yellow spot channels.
- Deleting a channel from an RGB, CMYK, or Lab image automatically converts the image to Multichannel mode, flattening layers.
- To export a multichannel image, save it in Photoshop DCS 2.0 format.
Note: Indexed Color and 32-bit images cannot be converted to Multichannel mode.
Applying colors to artwork is a common Adobe Illustrator task, and one that requires some knowledge of color models and color modes. When applying color to artwork, keep in mind the final medium in which the artwork will be published, so that you can use the correct color model and color definitions. Experimenting and applying color is easy using the feature-rich Swatches panel, Color Guide panel, and Edit Colors/Recolor Artwork dialog box in Illustrator.
Color models describe the colors we see and work with in digital graphics. Each color model, such as RGB, CMYK, or HSB, represents a different method for describing and classifying color. Color models use numeric values to represent the visible spectrum of color. A color space is a variant of a color model and has a specific gamut (range) of colors. For example, within the RGB color model are a number of color spaces: Adobe® RGB, sRGB, and Apple® RGB. While each of these color spaces defines color using the same three axes (R, G, and B), their gamuts are different.
About colors in digital graphics
We use color models to describe the colors we see and work with in digital graphics. Each color model, such as RGB, CMYK, or HSB, represents a different method for describing and classifying color. Color models use numeric values to represent the visible spectrum of color. A color space is a variant of a color model and has a specific gamut (range) of colors. For example, within the RGB color model are a number of color spaces: Adobe® RGB, sRGB, and Apple® RGB. While each of these color spaces defines color using the same three axes (R, G, and B), their gamuts are different.
When you work with the colors in a graphic, you are actually adjusting numerical values in the file. It’s easy to think of a number as a color, but these numerical values are not absolute colors in themselves—they only have a color meaning within the color space of the device that is producing the color.
Because each device has its own color space, it can reproduce colors only in its gamut. When an image moves from one device to another, image colors may change because each device interprets the RGB or CMYK values according to its own color space. For example, it is impossible for all the colors viewed on a monitor to be identically matched in a print from a desktop printer. A printer operates in a CMYK color space, and a monitor operates in an RGB color space. Their gamuts are different. Some colors produced by inks cannot be displayed on a monitor, and some colors that can be displayed on a monitor cannot be reproduced using inks on paper.
Even though it is impossible to perfectly match all colors on different devices, you can use color management to ensure that most colors are the same or similar enough so they appear consistent.
See also
RGB, CMYK, HSB, and Lab color models
RGB
A large percentage of the visible spectrum can be represented by mixing red, green, and blue (RGB) colored light in various proportions and intensities. Where the colors overlap, they create cyan, magenta, and yellow.
RGB colors are called additive colors because you create white by adding R, G, and B together—that is, all light is reflected back to the eye. Additive colors are used for lighting, television, and computer monitors. Your monitor, for example, creates color by emitting light through red, green, and blue phosphors.
Additive colors (RGB)
- R.
- Red
- G.
- Green
- B.
- Blue
You can work with color values using the RGB color mode, which is based on the RGB color model. In RGB mode, each of the RGB components can use a value ranging from 0 (black) to 255 (white). For example, a bright red color might have an R value of 246, a G value of 20, and a B value of 50. When the values of all three components are equal, the result is a shade of gray. When the value of all components is 255, the result is pure white; when all components have values of 0, the result is pure black.
Illustrator also includes a modified RGB color mode called Web Safe RGB, which includes only those RGB colors that are appropriate for use on the web.
See also
CMYK
Whereas the RGB model depends on a light source to create color, the CMYK model is based on the light-absorbing quality of ink printed on paper. As white light strikes translucent inks, a portion of the spectrum is absorbed. Color that is not absorbed is reflected back to your eye.
Combining pure cyan (C), magenta (M), and yellow (Y) pigments would result in black by absorbing, or subtracting, all colors. For this reason they are called subtractive colors. Black (K) ink is added for better shadow density. (The letter K came into use because black is the “key” color for registering other colors, and because the letter B also stands for blue.) Combining these inks to reproduce color is called four-color process printing.
Subtractive colors (CMYK)
- C.
- Cyan
- M.
- Magenta
- Y.
- Yellow
- K.
- Black
You can work with color values using the CMYK color mode, which is based on the CMYK color model. In CMYK mode, each of the CMYK process inks can use a value ranging from 0 to 100%. The lightest colors are assigned small percentages of process ink colors; darker colors have higher percentage values. For example, a bright red might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In CMYK objects, low ink percentages are closer to white, and high ink percentages are closer to black.
Use CMYK when preparing a document to be printed using process inks.
See also
HSB
Based on the human perception of color, the HSB model describes three fundamental characteristics of color:
- Hue
- Color reflected from or transmitted through an object. It is measured as a location on the standard color wheel, expressed as a degree between 0° and 360°. In common use, hue is identified by the name of the color, such as red, orange, or green.
- Saturation
- Strength or purity of the color (sometimes called chroma). Saturation represents the amount of gray in proportion to the hue, measured as a percentage from 0% (gray) to 100% (fully saturated). On the standard color wheel, saturation increases from the center to the edge.
- Brightness
- Relative lightness or darkness of the color, usually measured as a percentage from 0% (black) to 100% (white).
HSB color model- H.
- Hue
- S.
- Saturation
- B.
- Brightness
See also
Lab
The CIE Lab color model is based on the human perception of color. It is one of several color models produced by the Commission Internationale d’Eclairage (CIE), an organization dedicated to creating standards for all aspects of light.
The numeric values in Lab describe all the colors that a person with normal vision sees. Because Lab describes how a color looks rather than how much of a particular colorant is needed for a device (such as a monitor, desktop printer, or digital camera) to produce colors, Lab is considered to be a device-independent color model. Color management systems use Lab as a color reference to predictably transform a color from one color space to another color space.
In Illustrator, you can use the Lab model to create, display, and output spot color swatches. However, you cannot create documents in Lab mode.
See also
Grayscale
Grayscale uses tints of black to represent an object. Every grayscale object has a brightness value ranging from 0% (white) to 100% (black). Images produced using black-and-white or grayscale scanners are typically displayed in grayscale.
Grayscale also lets you convert color artwork to high-quality black-and-white artwork. In this case, Adobe Illustrator discards all color information in the original artwork; the gray levels (shades) of the converted objects represent the luminosity of the original objects.
When you convert grayscale objects to RGB, the color values for each object are assigned that object’s previous gray value. You can also convert a grayscale object to a CMYK object.
See also
Color spaces and gamuts
A color space is a range of colors in the visible spectrum. A color space can also be a variant of acolor model. Adobe RGB, Apple RGB, and sRGB are examples of different color spaces based on the same color model.
Gamuts of different color spaces
- A.
- Visual gamut
- B.
- RGB color space
- C.
- CMYK color space
The range of color encompassed by a color space is called a gamut. The different devices (computer monitor, scanner, desktop printer, printing press, digital camera) throughout your workflow operate within different color spaces and each with different gamuts. Some colors within the gamut of your computer monitor are not within the gamut of your inkjet printer, and vice versa. When a color cannot be produced on a device, it’s considered to be outside the color space of that particular device. In other words, the color is out of gamut.
See also
About spot and process colors
You can designate colors as either spot or process color types, which correspond to the two main ink types used in commercial printing. In the Swatches panel, you can identify the color type of a color using icons that appear next to the name of the color.
When applying color to paths and frames, keep in mind the final medium in which the artwork will be published, so that you apply color using the most appropriate color mode.
If your color workflow involves transferring documents among devices, you may want to use a color-management system (CMS) to help maintain and regulate colors throughout the process.About spot colors
A spot color is a special premixed ink that is used instead of, or in addition to, process inks, and that requires its own printing plate on a printing press. Use spot color when few colors are specified and color accuracy is critical. Spot color inks can accurately reproduce colors that are outside the gamut of process colors. However, the exact appearance of the printed spot color is determined by the combination of the ink as mixed by the commercial printer and the paper it’s printed on, not by color values you specify or by color management. When you specify spot color values, you’re describing the simulated appearance of the color for your monitor and composite printer only (subject to the gamut limitations of those devices).
Keep the following guidelines in mind when specifying a spot color:
- For best results in printed documents, specify a spot color from a color-matching system supported by your commercial printer. Several color-matching system libraries are included with the software.
- Minimize the number of spot colors you use. Each spot color you create will generate an additional spot color printing plate for a printing press, increasing your printing costs. If you think you might require more than four colors, consider printing your document using process colors.
- If an object contains spot colors and overlaps another object containing transparency, undesirable results may occur when exporting to EPS format, when converting spot colors to process colors using the Print dialog box, or when creating color separations in an application other than Illustrator or InDesign. For best results, use the Flattener Preview or the Separations Preview to soft proof the effects of flattening transparency before printing. In addition, you can convert the spot colors to process colors by using the Ink Manager in InDesign before printing or exporting.
- You can use a spot color printing plate to apply a varnish over areas of a process color job. In this case, your print job would use a total of five inks—four process inks and one spot varnish.
About process colors
A process color is printed using a combination of the four standard process inks: cyan, magenta, yellow, and black (CMYK). Use process colors when a job requires so many colors that using individual spot inks would be expensive or impractical, as when printing color photographs.
Keep the following guidelines in mind when specifying a process color:
- For best results in a high-quality printed document, specify process colors using CMYK values printed in process color reference charts, such as those available from a commercial printer.
- The final color values of a process color are its values in CMYK, so if you specify a process color using RGB (or LAB, in InDesign), those color values will be converted to CMYK when you print color separations. These conversions differ based on your color-management settings and document profile.
- Don’t specify a process color based on how it looks on your monitor, unless you are sure you have set up a color-management system properly, and you understand its limitations for previewing color.
- Avoid using process colors in documents intended for online viewing only, because CMYK has a smaller color gamut than that of a typical monitor.
- Illustrator and InDesign let you specify a process color as either global or non-global. In Illustrator, global process colors remain linked to a swatch in the Swatches panel, so that if you modify the swatch of a global process color, all objects using that color are updated. Non-global process colors do not automatically update throughout the document when the color is edited. Process colors are non-global by default. In InDesign, when you apply a swatch to objects, the swatch is automatically applied as a global process color. Non-global swatches are unnamed colors, which you can edit in the Color panel.
Note: Global and non-global process colors only affect how a particular color is applied to objects, never how colors separate or behave when you move them between applications.
Using spot and process colors together
Sometimes it’s practical to use process and spot inks in the same job. For example, you might use one spot ink to print the exact color of a company logo on the same pages of an annual report where photographs are reproduced using process color. You can also use a spot color printing plate to apply a varnish over areas of a process color job. In both cases, your print job would use a total of five inks—four process inks and one spot ink or varnish.
In InDesign, you can mix process and spot colors together to create mixed ink colors.
Comparing colors in InDesign and Illustrator
Adobe InDesign and Adobe Illustrator use slightly different methods for applying named colors. Illustrator lets you specify a named color as either global or nonglobal, and InDesign treats all unnamed colors as nonglobal, process colors.
The InDesign equivalents to global colors are swatches. Swatches make it easier to modify color schemes without having to locate and adjust each individual object. This is especially useful in standardized, production-driven documents like magazines. Because InDesign colors are linked to swatches in the Swatches panel, any change to a swatch affects all objects to which a color is applied.
The InDesign equivalents to nonglobal swatches are unnamed colors. Unnamed colors do not appear in the Swatches panel, and they do not automatically update throughout the document when the color is edited in the Color panel. You can, however, add an unnamed color to the Swatches panel later.
Named and unnamed colors only affect how a particular color updates in your document, never how colors separate or behave when you move them between applications.
Differences between RGB and CMYK Color Modes
When a user generates graphics on a computer for printing, or wishes to print images from a digital camera, it is a common mistake to assume that the colors seen on the screen will look the same in print. As a result of this mistake, files for printing are often erroneously sent in the Red-Green-Blue (RGB) format for printing. The issue lies in the fact that the computer screen and many photo editing programs show colors in RGB mode, while images are printed on paper in Cyan-Magenta-Yellow-Black (CMYK) format. Sometimes the conversion from RGB to CMYK works without any problems arising, and a printout will look identical to what shows up on the computer. In other cases, there will be noticeable differences between the shades of color. The key to avoiding this potential problem is to convert all graphics to CMYK format during the layout design phase.RGB Color Mode
RGB is the color scheme that is associated with electronic displays, such as CRT, LCD monitors, digital cameras and scanners. It is an additive type of color mode, that combines the primary colors, red, green and blue, in various degrees to create a variety of different colors. When all three of the colors are combined and displayed to their full extent, the result is a pure white. When all three colors are combined to the lowest degree, or value, the result is black. Software such as photo editing programs use the RGB color mode because it offers the widest range of colors.
CMYK Color Mode
Printers print color onto paper using the CMYK color mode only. This is a four color mode that utilizes the colors cyan, magenta, yellow and black in various amounts to create all of the necessary colors when printing images. It is a subtractive process, which means that each additional unique color means more light is removed, or absorbed, to create colors. When the first three colors are added together, the result is not pure black, but rather a very dark brown. The K color, or black, is used to completely remove light from the printed picture, which is why the eye perceives the color as black.
Why RGB Colors Need to be Converted
The RGB scheme has a greater range of colors than CMYK and can produce colors that are more vivid and vibrant. These colors are beyond the range of CMYK to reproduce and will come out darker and more dull in print than what is seen on the monitor or display. Because the RGB color mode has the full range of colors, documents shown in CMYK mode will always show up precisely on-screen. RGB colors, however, will not necessarily appear in print as they do on-screen. To accurately print the document or image, it must be converted from its original RGB format to CMYK. It is possible to do this by using software such as Adobe Photoshop or Adobe Illustrator.
Adobe Photoshop vs Adobe Illustrator
Adobe Photoshop is a complex and powerful photo and raster graphics editing program. It is designed for both simple and advanced photo retouching and editing. It is more ideally suited for web pages and the distribution of professional photographs in online galleries. Because Photoshop documents are generated in raster format, they do not scale very well. A Photoshop image that looks crisp and clean on an 8.5 inch by 11 inch document may look blocky, or pixelated, on a billboard. Furthermore, Photoshop images default in RGB format, which potentially means the colors seen in the document won't be precisely reproduced in print. Adobe Illustrator is a vector graphics editor that many graphics artists use to produce commercial quality logos and graphics, such as stationery, business cards and printed newsletters. Illustrator documents default to CMYK mode, which is the same color scheme that appears on paper. Also, because of its use of vector-based graphics, documents produced by Illustrator are safer for resizing. This means that an Illustrator document can theoretically look as crisp and clean on a billboard as it will on a business card. As a result, Adobe Illustrator is geared more for printed documents than Adobe Photoshop.
Converting RGB to CMYK
Converting RGB format pictures to CMYK mode for printing can be a complex task. It often involves using RGB to CMYK conversion tables, or various software utilities, to ensure that the colors seen on the computer screen will match the colors that appear on print. One way of making this process quicker and more efficient is to simply convert a file to CMYK format as soon as it is created if there are any future plans for converting the documents to printed format. Photographs that are imported into the document typically come in RGB mode, and it will be necessary to convert them individually. These can be problematic because they occasionally contain colors that are “out of gamut”, which means it contains colors that the CMYK color mode cannot reproduce. To check for this, click on the “View” menu in Photoshop and then “Gamut Warning”. Colors that turn gray are the ones that are incompatible with CMYK mode. Photoshop can either replace these colors with what it calculates as the next best color, or the user can use the color replace tool to select the color and manually replace it with the closest match. Once this is completed, the resulting document can be saved in CMYK mode and exported to Adobe Illustrator or Adobe InDesign, and later sent to a printer or printing service, with the confidence that the document will look the same on paper as it does on-screen.
Adapted from Photoshop CS5 Bible (Wiley Publishing)
By Lisa DaNae Dayley, Brad Dayley
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Photoshop provides several color modes that help when working with images. Ultimately, color is just different intensities of light at different frequencies. However, that data needs to be translated into a quantifiable form that can be understood by Photoshop, the monitor, printers, and ultimately you.
What follows gives you an overview of the different color modes available for use in Photoshop and why to use them. It also helps you understand the bits/channel settings for the image modes.
Understanding the Different Color Modes
A color model is simply a method to translate the light captured in an image into a digital form that the computer and other devices can understand. Each color model breaks the light into one or more channels and then assigns an intensity level of each channel for each pixel in the image. Photoshop provides several color modes that match the most common color models.
Depending on what you are doing with an image, you want to use a specific color mode that provides the best management of the color. To set the color mode of an image, select Image > Mode and then select the mode you want to use from the main menu in Photoshop. The following sections discuss each of the color modes and what they are for.
Bitmap
The bitmap color mode contains only one channel with only two possible levels, 0 and 255. The translation is that a bitmap is a black and white image without color and even without shades of gray. In essence, the image becomes a series of black dots on a white background. This may not sound very useful. However, there are several good uses for these types of images.
The most common use of the bitmap mode is outputting the image to a black and white laser printer. Laser printers create images as a series of black dots on the page. So working with the image as a bitmap lets you make changes to a version that appears exactly how it will when it is printed by the laser printer.
Tip: Bitmap images that are printed on low-resolution laser printers often end up darker than you would expect. Be sure to lighten the image before printing it to a low-resolution laser printer.
The bitmap dialog box shown below lets you set the resolution and method to use when creating the bitmap from the image.
 The options for the bitmap mode in Photoshop. |
You should set the resolution to the same resolution that you will be using to output the image to the printer. The following methods can be selected from the drop-down list in the dialog box:
- 50% Threshold: This sets every pixel in the image that is more that 50% gray to black and every pixel that is 50% gray or lighter to white. This is by far the simplest pattern; however, the end effect is very choppy, and the image typically doesn’t look very good unless you are trying for a special effect, for which it would be better to use the Threshold tool in the Image > Adjustment menu.
- Pattern Dither: The pattern dither uses a pattern to mix black and white pixels together, which results in the appearance of different shades of gray. The problem with this method is that the pattern shows up in the image, so the effect is not very smooth.
- Diffusion Dither: This uses an error-diffusion method of converting an image into a series of dithered pixels that are less structured than the pattern dither. The diffusion dither method produces an image similar to a mezzotint, which often is the best option for printing on low-resolution laser printers.
- Halftone Screen: This uses a series of dots of varying sizes and spacing that trick the eye into believing it is seeing a continuous tone. When you select the halftone screen option, an additional dialog box pops up that enables you to select the frequency, angle, and shape of the halftone dots. Typically, the best shape to use is the round shape because it is closest to what the printer generates. The frequency depends on the resolution of the printer. The higher the frequency, the better resolution you get in the image; however, if you set the frequency too high, the patterns overlap when printed and the results do not look good.
- Custom Pattern: This allows you to select a custom pattern, either one of those included with Photoshop or one of your own. The custom pattern can be used with similar results to the pattern dither. Typically, this option is used only if you have a specific pattern that you want to show up in the image.
Grayscale
The grayscale mode contains only one channel, but unlike the bitmap mode it can have intensity levels from 0 to 255. The grayscale mode is useful if you are outputting the image to a noncolor printer.
Tip: You can create a grayscale image from a single RGB channel by selecting that channel in the Channels panel and converting the image to grayscale. Only the selected channel is converted to grayscale. This can be useful for special effects as well as utilizing the detail of a specific channel.
Another advantage of grayscale is that viewing and adjusting the image in grayscale reduces the overhead of dealing with three color channels. Photoshop is much faster at performing complex operations on grayscale images than on multichannel images. Having a single channel also makes it easier to make adjustments to an image because you only need to worry about adjusting that channel.
Caution: To change an image from color to grayscale, Photoshop takes a composite intensity for all three channels and reduces it to the single grayscale channel. This results in a loss of the original color channels. Therefore, make sure you have a backup copy of the file before you save it again.
Duotone
The duotone mode uses on contrasting color of ink over another to produce highlights and middle tones in a black and white image. Duotones typically are used to prepare images for printing. Using the Duotones Options dialog box, you can add one, two, three, or four inks to create a monotone, duotone, tritone, or quadtone image in Photoshop.
Use the following steps to configure the monotone, duotone, tritone, or quadtone options from the Duotone Options dialog box shown below:
  The options for the Duotone color mode and the Overprint Colors dialog box in Photoshop. |
Note: The Duotone color mode option is available only for grayscale images. If you are using a color image you need to convert it to grayscale before changing to duotone.
- Select the type of tone from the Type menu.
- Click the blank swatch for each ink you need to specify.
- Select the color to use for that tone.
Typically, you should use black for the first ink in the list. - Click the curve for each ink you need to configure to launch the Duotone Curve tool.
- Use the Duotone Curve tool to adjust the tone curve for that color of ink.
Typically, you do not need to adjust the curve unless you want a specific effect from that tone of ink. - Click the Overprint Colors button, shown above, to bring up the Overprint Colors dialog box.
- Adjust the colors used when one ink is printed over another ink by selecting the colors for each of the overlapping options.
Indexed Color
The indexed color mode contains a single channel with a single set of indexed colors. Converting an image to indexed color reduces the image to a set of most important colors. The remaining colors are given an index between 3 and 256 in a color lookup table. Instead of using the level value of the pixel to define the intensity of the channel, it points to an index value in the lookup table for the indexed image.
To change the color mode of an image to indexed color, select Image > Mode > Indexed Color from the main menu. To view the table of indexed colors, select Image >Mode > Color Table from the main menu to bring up the Color Table dialog box shown below. The following are settings that can be modified in the Indexed Color dialog box:
  The options for the Indexed color mode and the Color Table dialog box in Photoshop. |
- Palette: This lets you select which palette to use when choosing the color to be placed in the index. You can choose a palette based on Exact, System, Web, Uniform, Local, Master, or Custom colors. The Local option selects colors local to the image. If you are working with multiple images, the Master option selects colors from a master of all images. When selecting local or master palettes, you can use a Selective, Adaptive, or Perceptual method of choosing the colors. The Selective method tries to preserve the key colors in the image. The Adaptive method simply preserves the most common colors. The Perceptual method intelligently selects colors that will provide the best transitions, rather than just the most popular ones. You also have the option to select System colors for images that will be viewed only on a computer. The Web option selects only Web-safe colors for images that will be used on Web pages.
- Colors: This specifies the number of colors to use in the color lookup table. The minimum is 3 colors and the maximum is 256.
Tip: When creating Web images with file formats such as .gif and .png, using fewer colors results in smaller images. - Forced: This lets you force the conversion to keep certain colors in the image. The default is to force only black and white to be kept. The Primaries option protects eight colors: white, black, red, green, blue, cyan, yellow, and magenta. The Web option protects the 216 colors in the Web-safe colors. The Custom option allows you to preserve a specific palette of colors that you create. When you select the Custom option, a dialog box launches that allows you to specify the colors that you want to preserve.
- Transparency: This specifies whether to preserve the transparency in the image.
- Matte: The matte option allows you to specify a matte to use when working with transparency in the image. If there is no transparency in the image, this option is inactive. If the transparency option is selected, the translucent areas in the image are filled with the matte color. If the transparency option is deselected, the translucent and transparent areas are filled with the matte color.
- Dither: This specifies the method Photoshop uses to calculate replacement colors for colors being discarded from the image. The None option simply selects the closest color in the lookup table, which can sometimes result in harder edges but is typically the best option to use. The Diffusion option dithers the color randomly, creating a more naturalistic effect. The Pattern option dithers in geometric patterns, which is usually the least desirable because the patterns show up in the image. The Noise option mixes pixels throughout the image instead of just the areas of transition.
- Amount: This specifies the percentage of diffusion to use when dithering. This option is available only when you select the Diffusion dithering option. Lower values decrease the size of the file but result in harsher color transitions.
- Preserve Exact Colors: This turns dithering off for areas of solid color when the Diffusion dithering option is selected. This option is not available unless the Diffusion dithering option is selected. Using this option helps your images look better even if you have to use dithering.
RGB Color
You likely will use the RGB color model most often. The RGB color model is used by computer monitors and the human eye. Photoshop processes its wide range of vivid colors quickly. RGB is the model that most closely matches the human eye for two reasons. First, RGB uses three colors similar to the receptors in the human eye. Second, RGB is additive, meaning that as you add more color, you get more light, in the same way that more color results in the eye seeing white.
In the RGB model, colors are divided into three channels of red, green, and blue. Each channel has an intensity level range between 0 and 255. Each color is made up of a combination of intensity levels from these three channels, resulting in the possibility of over 16.7 million different colors.
RGB provides by far the most vibrant use of colors of all the color models and is supported by most file formats. The one downside to RGB is that it contains more colors, especially the brighter ones, than can be printed. This can result in loss of detail in areas of your images when they are printed. The solution is to use the CYMK color model if you are going to have your images professionally printed.
CYMK Color
The CYMK color model is completely different from the RGB model in that it uses a subtractive method, meaning that the more color is added, the less light is seen. This is one of the reasons the CYMK model works so well for printing. Think about adding ink to a page; if you add all the colors, you get black, or rather a really deep brown.
Another difference between the CYMK model and the RGB model is that it is made up of four channels: cyan, yellow, magenta, and black. The black channel is necessary because adding the ink all together makes a dark brown not black, so if you want the printer to print true black, you must have a separate channel to specify black.
Which model should you use for general color image editing? The answer is RGB. The RGB model provides the widest range for tonal adjustment and correction. The scanner, monitor, and most other devices (except printers) work in the RGB model. Also, editing images in Photoshop in the RGB mode is much faster than in CYMK.
Tip: Even if you are using the RGB model, you can select View ? Proof Colors from the main menu to toggle the view to a simulated CYMK model. This way you can periodically check to see how the image will look when printed. Just remember to turn it off.
Lab Color
The Lab color model is very different from RGB and CYMK. The Lab model does have three channels, but instead of all three dedicated to colors, only two—a and b—are dedicated to color; the third—Lightness—is dedicated to luminosity.
The a channel maps colors ranging from deep green at level 0 to gray at level 127 to a rich pink at 255. The b channel maps colors ranging from bright blue at level 0 to gray at level 127 to a dim yellow at 255. The Luminosity channel maps the brightness of each pixel from dark at 0 to white at 255.
The Lab channel is additive like the RGB model, but it has only two channels of color mixing, and the levels of those channels are not mapping to intensity but rather tones of color. The tones add together to form brighter colors, and only the luminosity channel provides data to darken the tone that is created by the other two channels.
Editing images in Lab color is about the same speed as RGB and much faster than CYMK, so it is a fun alternative if you want to adjust your thinking of mixing colors.
Multichannel
The Multichannel mode separates out the channels in the current color model into spot channels. Spot channels can be used to store parts of an image that you want to print in specific inks or spot colors. For example, you can print specific inks from a Pantone library.
When you convert an image to the Multichannel model, the current channels are changed to spot channels. The channels created in Multichannel mode depend on the original color mode of the image. For example, the RGB mode gets converted to cyan, magenta, and yellow spot channels, the CYMK model gets converted into cyan, magenta, yellow, and black spot channels, and the Lab model gets converted into three Alpha channels.
Note: The spot channels overlap, so if you do not want ink from one channel to be printed on ink from another channel, the data in those areas of the channel cannot overlap.
Bits Per Channel
How many bits should you use per channel? The quick answer is 8 bits per channel, but let’s look a bit closer. What does bits per channel mean? A bit is a single item of information for a computer with a value of 0 or 1. That doesn’t mean much in terms of an image, but if you string millions of bits together, it can mean a lot.
Using 8 bits of information, we can define an intensity level of 0 to 255. For three channels, we can define about 16.7 million different colors for each channel. If we use 16 bits per channel, that goes up to over 281.4 trillion colors; if we go to 32 bits per channel, well, you get the idea.
So why not just use 32 bits per channel and maximize our information? The answer is disk space and speed. An image with 32 bits per channel takes up much more disk space and much more effort to edit on Photoshop’s part. Plus, the human eye can’t even detect all the colors in the 8 bits per channel.
And that leads to the question of why not just use 8 bits per channel, because it is more than enough for the human eye? The answer lies in what happens during adjustments, corrections, and conversions. Each time you make a correction to an image, change the levels, add a filter, and so on, you lose a little bit of the distinguishing detail. If you do enough corrections on an image with 8 bits per channel, you may lose noticeable detail in the image. However, if you are using an image with 16 bits per channel, the data lost is in levels that cannot be detected by the human eye, so when you convert the image back to 8 bits per channel, there is no data loss.
An image must be in the RGB or Lab color modes to convert it to 16 bits per channel. To change your image to 16 bits per channel, select Image > Mode > 16 Bits/Channel. After you have changed your image to 16 bits per channel, you can change it to 32 bits per channel by selecting Image > Mode > 32 Bits/Channel.
Note: An image with 32 bit per channel is considered an HDR (High Dynamic Range) image. Typically, these images are used in 3D rendering and advanced CGI animation effects.
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