The Helmholtz theory' of colour vision states tha
(A) There are three kinds of cones in the retina responding to the three primary colours > Hermann von Helmholtz developed the theory further in 1850: that the three types of cone photoreceptors could be classified as short-preferring (blue), 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 blue, medium green, and low red.> 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.
There are three kinds of cones in the retina responding to the three primary colours
There are two kinds of cones called dominators and modulators.
There is only one kinds of cone and the colour is recognised only in area 17.
There are seven types of cones responding to the seven colours of the spectrum.
Special Senses: Vision and its Elements