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Chapter 13: Color
Computer Color
Computer monitors display colors using varying amounts of red, green, and blue, called RGB color. This is considered an additive form of color, because red, green, and blue light in equal amounts "add" up to white light. All other colors are formed on screen by varying the amounts of each color. For example, red and green are combined to form yellow, blue and green to form cyan, and red and blue to form magenta. This is demonstrated at

RGB color is completely different from the way colors are set in print. In print, CMYK (cyan, magenta, yellow, black) is the more common color scheme. The colors you see on a printed piece are the parts of the spectrum reflected back to your eyes as white light hits the ink. CMYK is considered subtractive color since, in theory, if you were to mix pure cyan, magenta, and yellow, they should absorb all color to produce black (because of impurities in all printing inks, however, these three don't actually produce black, which is why black (K) ink must be added).

Each of the various dots or pixels (picture elements) on a computer monitor is set to a particular color to create imagery on screen. Bit-depth, sometimes referred to as color depth, is the term given to the number of bits used to describe color in an image or on a monitor. The basic idea is simple: the more bits used to specify a color, the more possible colors are available—more bits equals more colors. One bit can be used to specify two colors, typically black and white, two bits can describe four colors, three bits can describe eight colors, and so on. Notice that the number of colors specified by the bit-depth is simply 2 raised to the nth power, where n is the number of bits, as shown in Table 13-2.

Bits Number of possible colors
32 16,777,216 (24 bits) plus 8 bits used for control information.
24 16,777,216
16 65,536
8 256
7 128
6 64
5 32
4 16
3 8
2 4
1 2
Table 2: Bit Depth and Possible Colors

A firm understanding of bit depth is important for a Web designer, as the bit depth of a visitor's monitor affects color reproduction, and the manipulation of bit-depth in images can be used to significantly decrease file size. Put simply, the higher the bit depth, the greater the number of colors; the greater the number of colors, the larger the file size. Reducing the number of colors will aid in lowering the file size of images and improve the download time of Web pages. Further consideration of this idea is given in Chapter 14.

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