Color opponency and discrimination. 
The rationale for color opponency is investigated.  A principle of optimal discrimination of visual information is introduced: If two visual variables are kept fixed, the third one, left free, must vary as much as possible.  We prove that, if satisfied, the principle requires the existence of two chromatic functions, which are under the reference source E (corresponding to the equal-energy spectral distribution function), orthogonal with respect to a certain inner product, so that given one function, the other one is determined, apart from a multiplicative constant.  Experimental chromatic functions are recovered.  Two almost-illuminant-independent chromatic variables, eta and zeta, are constructed from the achromatic variables in the companion paper.