Since König, experiment has yielded 6 different sets of 3 “primary processes” for tricolor vision in the fovea. Their variation with test method, and conflict with other data, suggest only an indirect relation to the absorption curves of receptors. Blue and yellow in the retinal periphery, in the central fovea, and in transient effects, act as if coupled physiologically. Yellow proves independent of red and green, and blue has many characteristics of rod function.
To adjust the Helmholtz view of deficiency to modern observations, new mechanisms for white and hues are needed in the cortex. The Müller theory adds black and locates in the retina this inner zone; but likewise fails to assign it a physiological mechanism. Recent experiments confirm the retinal location of both yellow and black mechanisms; hence an opponents theory seems unavoidable. The psychophysical evidence calls for unsymmetric synaptic connections before the “primary processes,” and also after them to create hues and luminosity separately.
S. A. TALBOT, "Recent Concepts of Retinal Color Mechanism. I. Contributions from Psychophysics," J. Opt. Soc. Am. 41, 895-917 (1951)