While Newton investigated the physical nature of light and Goethe contributed to the understanding of the psychology of color vision, Thomas Young, a physician and a physicist, added to the understanding of the neurological aspects of color vision. Young, who had made some significant contributions in the field of light waves, claimed that the human eye could not possibly contain a separate mechanism for every perceptible color. He concluded that as it is possible to produce any color by mixing red, green and blue light waves in different quantities, mechanisms for discerning light of these colors would be sufficient to enable perception of all colors. From this he inferred that the human eye contains only three types of light receptors, called photoreceptors, each of which is sensitive to a different color. These are the primary colors.

Young's theory was further refined, towards the close of the century, by the research of the German physicist Herman L. F. von Helmholtz, and is known today as the Young - Helmholz Theory. This theory defines a trichromatic system of the three primary colors - red, green and blue (violet) whereby a mixture of any two colors creates a different (secondary) color, and  a mixture of all three creates white.