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The four-color ribbon comprises of four inks used in printing: three secondary colors plus black. These ink colors are cyan, magenta and yellow; abbreviated as CMYK. Cyan can be thought of as minus-red, magenta as minus-green, and yellow as minus-blue. These inks are semi-transparent or translucent. Where two such inks overlap on the paper due to sequential printing impressions, a primary color is perceived. For example, yellow (minus-blue) overprinted by magenta (minus green) yields red. Where all three inks may overlap, almost all incident light is absorbed or subtracted, yielding near black. It is because of this poor "subtractive" black that a separate black ink is used. The secondary or subtractive colors cyan, magenta and yellow may be considered "primary" by printers and watercolorists (whose basic inks and paints are transparent).
Reference: http://en.wikipedia.org/wiki/Color_printing
