CMYK is a particular color model based on four primary colors: cyan (bluish-green), magenta, yellow and key (black). Occasionally pronounced “c-mike,” CMYK may also be written as YMCK or CYM. The four primary colors of the CMYK color system can be combined a variety of ways to create other secondary colors.
Using in printing color photographs, this subtractive color model works by rendering the color of an object through the light that isn’t absorbed. For example, lettuce appears green because it absorbs all colors except for cyan and yellow, the two colors that combine to produce the secondary color that the eye sees as “green.”
Although black isn’t considered to be a primary color in most color systems, it is included in the CMYK color model because the combination of each of the three primaries forms a murky brown, rather than a true black. Consequently, “key” was added to the system as the black that is characteristic of the “key plate,” the standard plate which is inked with only black.
RGB (red, green, blue) is an alternative to the CMYK color model. The system a photographer chooses will depend on the true colors he wants rendered in the resulting prints. For example, while an RGB system can render a “pure” blue (one that is 100 percent blue), the CMYK tends to produce more purplish blues. The color that is most appropriate depends on the subject and the photographer’s eye.