Process Color

Taken from WikiPedia:

Process color is a common shortened form of the term "four-color printing process." Process color (and similar terms) refers both to a method of reproducting colored images on printing presses and to the specific ink colors used. Other ways of referring to this printing process include "four color," "CMYK," "full process," and "full color."

The four-color printing process uses four standard ink colors: Three primary colors (cyan, magenta, and yellow), plus black (abbreviated as "K" for "key"). Using black ink provides shadow detail and reduces the amount of the primary colors needed to print dark hues.

The three process-color inks are primary colors. That is, the colors are theoretically capable of being combined to produce all other colors. Cyan is a bright bluish-green, magenta is a bright purplish-red, and yellow is a clear, bright yellow.