Cone cells are one of the two kinds of
photoreceptors in the eyes (the other being rods).
Cones, as you infer in your question, are
responsible for the perception of color. One thing
that should be clarified is that people don't
only have 3 cone cells, but rather have only 3
kinds of cone cells, and many of each kind
within the eye.
The 3 different kinds of cone cells people
typically have in their eyes are: S-cones,
M-cones, and L-cones. S-cones absorb wavelengths
of blue light, M-cones absorb green, and L-cones
absorb red. The differences in the types of cone
cells is the result of being comprised of
different kinds of photopsins, or photoreceptor
proteins that are typically receptors that undergo
a conformational change when activated by light.
So for the S-cone cells, blue light activates the
photopsins in the cells, whereas the M-cone
photopsins are activated by green light and the
L-cone photopsins are activated by red light. Once
these conformational changes take place in the
protein, a series of signals are sent throughout
the cone cell and eventually, all along the
pathway to the visual cortex in the brain, which
forms images based on the signals it received.
People who have 3 kinds of cone cells thus see
in something akin to the "RGB" (red-green-blue)
based colors we're used to seeing on a computer
screen. People with 4 different kinds of cones,
however, see in RGB + some variation/"shade" of R,
G, or B. Ultimately this corresponds having an
absorption peak at a slightly different wavelength
that still might correspond to some "shade" of R,
G, or B. But now, because of this extra
responsiveness to a fourth "color," there is a
larger number of combinations for the shades of
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