CMYK model
The CMYK model is based on the light-absorbing quality of ink printed on paper. As white light strikes translucent inks, certain visible wavelengths are absorbed (subtracted), while others are reflected back to your eyes. For this reason, these colors are called subtractive colors.
In theory, pure cyan (C), magenta (M), and yellow (Y) pigments should combine to absorb all light and produce black. Because all printing inks contain some impurities, however, these three inks actually produce a muddy brown. For this reason, black ink (K) is used in addition to the cyan, magenta, and yellow inks in four-color printing. (K is used for black instead of B to avoid confusion with blue.)
Subtractive colors (the CMYK model)