colors is subtractive (Cyan, Magenta and Yellow printed together on white result to black). CMYK works through light absorption. The colors that are seen are from the part of light that is not absorbed. In CMYK magenta plus yellow produces red, magenta plus cyan makes blue, cyan plus yellow generates green and the combination of cyan, magenta and yellow form black.
Because the 'black' generated by mixing the subtractive primaries is not as dense as that of a genuine black ink (one that absorbs throughout the visible spectrum), four-color printing uses a fourth, black, ink in addition to the subtractive primaries yellow, magenta and cyan.
Use of four-color printing generates a superior final printed result with greater contrast. However the color a person sees on a computer screen is often slightly different from the color of the same object on a printout since CMYK and the RGB color model used in computer monitors are different. RGB color is made by the reflectance or emission of light whereas CMYK works by the absorption of it.