CMYK color model

The CMYK color model is a subtractive color model, based on the CMY color model, used in color printing. CMYK refers to the four inks used in printing: cyan, magenta, yellow, and key (black).
The CMYK model is subtractive. It means that it "subtracts" or masks colors from white background of the paper. The ink reduces the reflected light. White light minus red leaves cyan, white light minus green leaves magenta, and white light minus blue leaves yellow.
White is the natural color of the paper, while black results from a full combination of colored inks. To save cost on ink, and to produce deeper black tones, unsaturated and dark colors are produced by using black ink instead of pure mix of cyan, magenta, and yellow.
Using black ink.
The "black" produced by mixing usual cyan, magenta, and yellow inks is actually a dark grey color. So, modern four-color printing uses black ink in addition to the subtractive primary colors. Common reasons for using black ink include: