A printing press uses a CMYK color model, in which three colors of transparent ink (cyan — C, magenta — M, and yellow — Y) are combined along with black (noted as K, derived from “key color”) in varying amounts to create different colors. CMYK inks filter the white light that reflects back from the paper and subtract some of the red, green, and blue light from the spectrum. The color we see is what’s left.
In theory, pure cyan, magenta, and yellow pigments should combine to absorb all color and produce black. But because all printing inks contain impurities, these three inks actually produce a muddy brown and must be combined with black ink to produce a true black. Combining these inks to reproduce color is called four-color process printing.