Reproducing colors
Colors on a monitor are displayed using combinations of red, green, and blue light (called RGB), while printed colors are typically created using a combination of four ink colors—cyan, magenta, yellow, and black (called CMYK). These four inks are called process colors because they are the standard inks used in the four-color printing process.
RGB image with red, green, and blue channels
CMYK image with cyan, magenta, yellow, and black channels
 | For color samples of channels in both RGB and CMYK images, see figure 18-1 and figure 18-2 in the color section. |
Because the RGB and CMYK color models use very different methods to display colors, they each reproduce a different gamut, or range of colors. For example, because RGB uses light to produce color, its gamut includes neon colors, such as those you’d see in a neon sign. In contrast, printing inks excel at reproducing certain colors that can lie outside of the RGB gamut, such as some pastels and pure black. For an illustration of the RGB and CMYK gamuts and color models, see figure 18-3, 18-4 and 18-5 in the color section.
But not all RGB and CMYK gamuts are alike. Each model of monitor and printer is different, and so each displays a slightly different gamut. For example, one brand of monitor may produce slightly brighter blues than another. The color space for a device is defined by the gamut it can reproduce.
RGB modelA 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, yellow, and white. Because the RGB colors combine to create white, they are also called additive colors. Adding all colors together creates white—that is, all light is transmitted back to the eye. Additive colors are used for lighting, video, and monitors. Your monitor, for example, creates color by emitting light through red, green, and blue phosphors. CMYK model The CMYK model is based on the light-absorbing quality of ink printed on paper. As white light strikes translucent inks, part of the spectrum is absorbed while other parts are reflected back to your eyes. In theory, pure cyan (C), magenta (M), and yellow (Y) pigments should combine to absorb all color and produce black. For this reason these colors are called subtractive colors. Because all printing inks contain some impurities, these three inks actually produce a muddy brown and must be combined with black (K) ink to produce a true black. (K is used instead of B to avoid confusion with blue.) Combining these inks to reproduce color is called four-color process printing. –From Adobe Photoshop 7.0 online Help |
An ICC profile is a description of a device’s color space, such as the CMYK color space of a particular printer. In this lesson, you’ll choose which RGB and CMYK ICC profiles to use. Once you specify the profiles, Photoshop can embed them into your image files. Photoshop (and any other application that can use ICC profiles) can then interpret the ICC profile in the image file to automatically manage color for that image. For general information about color management and about preparing your monitor, see Lesson 17, “Setting Up Your Monitor for Color Management”.
 | For information on embedding ICC profiles, see Photoshop 7.0 online Help. |