CMYK color model
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:
