Opponent Processing
nThe responses of the L,I,S cones are processed by three opponent channels: yellow-blue, red-green, and white-black. It is the latter information which is passed to the brain.
n        S           I         L    type of cones
nY-b       -            +        +     contribution to the signal
nR-g       +            -        +
nW-bl     +           +        +