Tetrachromacy
Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four different cones. Organisms with tetrachromacy are called tetrachromats. For these organisms, the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no fewer than four different pure spectral lights.

# Description
The normal explanation of tetrachromacy is that the organism's retina contains four types of higher-intensity light receptors (called cone cells in vertebrates as opposed to rod cells which are lower intensity light receptors) with different absorption spectra. This means the animal may see wavelengths beyond those of a typical human being's eyesight, and may be able to distinguish colors that to a human are identical.
The zebrafish (Danio rerio) is an example of a tetrachromat, containing cone cells sensitive for red, green, blue, and ultraviolet light.
Tetrachromacy is expected to occur in several species of birds, fish, amphibians, reptiles, arachnids and insects.

# Possibility of human tetrachromats
Humans and closely related primates normally have three types of cone cells and are therefore trichromats (animals with three different cones).  However, at low light intensities the rod cells may contribute to color vision, giving a small region of tetrachromacy in the color space. It has been suggested that women who are carriers for variant cone pigments might be born as full tetrachromats, having four different simultaneously functioning kinds of cones to pick up different colors. One study suggested that 23% of the world's women might have the kind of fourth cone that lies between the standard red and green cones, giving, theoretically, a significant increase in color differentiation. However, another study suggests that as many as 50% of women and 8% of men may have four photopigments.
Further studies will need to be conducted to verify tetrachromacy in humans. Two possible tetrachromats have been identified: "Mrs. M," an English social worker, was located in a study conducted in 1993, and an unidentified physician, also in England. Variation in cone pigment genes is widespread in most human populations, but the most prevalent and pronounced tetrachromacy would derive from female carriers of major red-green pigment anomalies, usually classed as forms of "color blindness" (protanomaly or deuteranomaly).  The biological basis for this phenomenon is X-inactivation.

# Difficulties
It is possible that some humans could have four rather than three color receptors.  Preliminary visual processing occurs within the nerves of the eye.  It is not known how these nerves would respond to a new color channel, if they could handle it separately or would just lump it in with an existing channel. Visual information leaves the eye by way of the optic nerve.  It is not known if the optic nerve has the spare capacity to handle a new color channel.  A variety of final image processing takes place in the brain.  It is not known how the various areas of the brain would respond if presented with a new color channel.

# Historical remarks
According to Lord Rayleigh in 1871, "Sir John Herschel even thinks that our inability to resolve yellow leaves it doubtful whether our vision is trichromatic or tetrachromatic..."