Ewald Hering (1834-1918) German physiologist who did much research into color and spatial perception. Hering disagreed with the leading theory developed mostly by Hermann von Helmholtz. Helmholtz's theory stated that the human eye perceived all colors in terms of three primary colors. (Red, Green, Blue).

Hering looked more at qualitative aspects of color and said there were six primary colors, coupled in three pairs: red-green, yellow-blue and white-black. His theory was rehabilitated in the 1970's when Edwin Land developed the Retinex theory that stated that whereas Helmholtz's colors hold for the eye, in the brain the three colors are translated into six.

Reference: R. Steven Turner, In the eye's mind : vision and the Helmholtz-Hering controversy (1994, Princeton University Press).