The YoungâHelmholtz theory (based on the work of Thomas Young and Hermann von Helmholtz in the 19th century), also known as the trichromatic theory, is a theory of trichromatic color vision â the manner in which the visual system gives rise to the phenomenological experience of color. In 1802, Young postulated the existence of three types of photoreceptors (now known as cone cells) in the eye, with different but overlapping response to different wavelengths of visible light. 
Hermann von Helmholtz developed the theory further in 1850:  that the three types of cone photoreceptors could be classified as short-preferring ( violet), middle-preferring ( green), and long-preferring ( red), according to their response to the wavelengths of light striking the retina. The relative strengths of the signals detected by the three types of cones are interpreted by the brain as a visible color.
For instance, yellow light uses different proportions of red and green, but little blue, so any hue depends on a mix of all three cones, for example, a strong red-sensitive, medium green-sensitive, and low blue-sensitive. Moreover, the intensity of colors can be changed without changing their hues, since intensity depends on the frequency of discharge to the brain, as a blue-green can be brightened but retain the same hue. The system is not perfect, as it does not distinguish yellow from a red-green mixture, but can powerfully detect subtle environmental changes. In 1857, James Clerk Maxwell used the recently developed linear algebra to offer a mathematical proof of the YoungâHelmholtz theory. 
The existence of cells sensitive to three different wavelength ranges (most sensitive to yellowish green, cyanish-green, and blue â not red, green and blue) was first shown in 1956 by Gunnar Svaetichin.  In 1983 it was validated in human retinas in an experiment by Dartnall, Bowmaker, and Mollon, who obtained microspectrophotopic readings of single eye cone cells.  Earlier evidence for the theory had been obtained by looking at light reflected from the retinas of living humans, and absorption of light by retinal cells removed from corpses. 
The direct proof that the eye does contain three types of cone has been secured, but only relatively recently. This was done by examining the light emerging from the eye after reflection off the retina; in the dark-adapted eye the light emerging was deficient in blue light because this had been preferentially absorbed by the rhodopsin. In the light-adapted eye, when only cone pigments are absorbing light, the emerging light can be shown to be deficient in red and green light because of the absorption by pigments called erythrolabe and chlorolabe. Again, the light passing through individual cones of the excised human retina can be examined by a microscope device, and it was shown by such examination that cones were of three different kinds according to their preference for red, green, and blue lights.