The CMYK color model is used in printing. The mixture of cyan, magenta
and yellow primary colors is subtractive when printed on a white piece
of paper and results to black. In reality, a perfect mixture of primary
colors contains certain impurities, which let an ideal mixture look dark
brown and not black. In printing, black is added as forth color for that
reason.
The CMYK color model works through light absorption. The colors that
are seen by an observer are from the part of light that is not absorbed
by the colors printed on a piece of paper, but reflected back to his eyes.
The following figure shows subtractive color mixing:
