Why is black-and-white so important in color?

In the visual system, the retina communicates with the brain by means of a black-white (achromatic) signal and two color-difference signals, a red-green and a yellow-blue. The existence of the black-white signal has important implications in imaging. First, if areas that are intended to be black. gray, or white are reproduced with even a slight tinge of hue, the defect is usually very noticeable, because these achromatic perceptions correspond to the color-difference signals being balanced at their null levels. Second the achromatic signal largely determines the apparent contrast of scenes, and their images only look correct if their gray scales are adjusted with due allowance for the effect of the surround on the black-white signal. Third, in luminance-chrominance television, important reductions in bandwidth are possible because of the lower sharpness required in the chrominance signals as a consequence of the greater number of cones necessary to generate the color-difference signals than to generate the black-white signal; the extent to which advantage is taken of this situation is discussed in connection with various forms of imaging.