The CMYK Color Gamut
CMYK color is all about ink on paper: Cyan, Magenta, Yellow and black (the Key plate to which the other colors register). CMYK color mixes are shown using percentages from 0 to 100 for each color.
Ink on paper is a reflective color environment. Instead of emitting color lights, paper surfaces absorb incoming light, acting as a filter to the color of light reflected back. Cyan, magenta and yellow inks are similar to the basic three color mixture of RGB. And black is often called the "punch" plate by printers because it adds contrast to the mix.
In order to print color images using inks, it was necessary to convert artwork (initially mostly photographs) into mechanical separations which would each carry one color of ink. To reproduce gradations of color, a series of dots in varying sizes, called half-tones, are used to deposit very small amounts of ink. The mixing of halftone screens at compatible angles allows the visual mixing of four ink colors, providing a large spectral range. Printers use CMYK inks with a variety of dot shapes for special effects.
Almost every mass production method that uses ink requires use of color separated artwork including offset printing, screen printing, and flexography.