CMYK

CMYK, or process color, is the term for the inks used to add colors to books. Books that use these inks are called four-color books because they use a combination of cyan, magenta, yellow, and black to create a design's desired color. Books containing color photography or colorful art will use CMYK when printing. Solid inks can be used in tandem with CMYK to make the color stand out.

Four color vs two color

As opposed to the versatility of four-color books, two-color books will not use CMYK and will instead only use black and another solid ink color.