Cyan is the complement of red. So, the cyan is a filter that does not allow red color through. The amount of cyan ink put on a white sheet of paper controls how much of the red (in white light) will be reflected back from the paper. Magenta is the complement of green (does not pass it), and yellow is the complement of blue (does not pass it). Combinations of different amounts of the three hues can produce a wide range of colors.