The color
opponent process is a color theory that states that the human
visual system interprets information about
color by processing signals from
cones and
rods in an antagonistic manner. The three types of cones (L for long, M for medium and S for short) have some overlap in the
wavelengths of
light to which they respond, so it is more efficient for the visual system to record
differences between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels:
red versus
green,
blue versus
yellow, and
black versus
white (the last type is
achromatic and detects light-dark variation, or
luminance). Responses to one color of an opponent channel are antagonistic to those to the other color. That is, opposite opponent colors are never perceived together – there is no "greenish red" or "yellowish blue".