Additive colors are produced through the addition of different colored light. In theory, every color can be produced by mixing the primary colors of the visible light spectrum: red, green and blue (RGB). Combining all three primary colors to equal parts produces white. The color vision of the human eye works through red-, green- and blue- sensitive sensory cells. When, for example, red and green rays of light reach the corresponding receptor cells in the eye, we see the mixed color yellow. When all three color receptors are stimulated, the eye sees white. RGB is the usual additive color system and is used predominantly for television screens, computer monitors and scanners. Combining two of the primary colors in equal parts produces the secondary colors cyan, magenta and yellow, which in turn form the basic colors of the subtractive color system (CMYK).