‘‘When it comes to deriving suitable and rigorous concepts and designations for the various characteristics of our sensations, the first requirement is that these concepts should be derived entirely out of the sensations themselves. We must rigorously avoid confusing sensations with their physical or physiological causes,or deducing from the latter any principle of classification’’
Ewald Hering, 1878

The following are excerpts from David Hubel’s book Eye, Brain, and Vision

“Hering’s theory of three opponent systems, for red-green, yellow-blue, and black-white, was regarded in his time and for the next half-century as rivaling and contradicting the Young-Helmholtz three-pigment (red, green, and blue)theory: the proponents of each were usually strongly partisan and often emotional. Physicists generally sided with the Young-Helmholtz camp, perhaps because their hearts were warmed by the quantitative arguments—by such things as linear simultaneous equations—and turned or cooled off by arguments about purity of colors…”

“In retrospect, as the contemporary psychophysicists Leo Hurvich and Dorothea Jameson have pointed out, it seems that one difficulty many people had with the Hering theory was the lack, until the 1950s, of any direct physiological evidence for inhibitory mechanisms in sensory systems. Such evidence became available only a half-century later, with single-unit recordings…Hering in some ways was fifty years ahead of his time….”

What amazes us today is that with so little to go on, Hering’s formulation turned out to describe cell-level central-nervous-system color mechanisms so well. Nevertheless, color-vision experts are still polarized into those who feel Hering was a prophet and those who feel that his theories represent a fortuitous fluke. To the extent that I am slightly to the Hering side of center, I will doubtless make enemies of all the experts.”

David Hubel, Nobel Laureate in Physiology, 1981
(from Eye, Brain and Vision, 1988)