Colour vision is the ability of an organism to distinguish objects based on the wavelengths.
In essence, different people see the same illuminated object or light source in different ways.
Nothing categorically distinguishes the visible spectrum of electromagnetic radiation from invisible portions of the broader spectrum. In this sense, colour is not a property of electromagnetic radiation, but a feature of visual perception by an observer. The possibility of a clean dissociation between colour experiences from properties reveals that colour is a subjective psychological phenomenon.
Today, most mammals possess dichromatic vision, corresponding to red–green colour blindness. They can thus see violet, blue, green and yellow light, but cannot see deep red light.
In most species of monkeys, males are dichromats, and about 60% of females are trichromats. Visual sensitivity differences between males and females in a single species is due to the gene for yellow-green sensitive opsin protein – which confers ability to differentiate red from green- residing on the X sex chromosome.
Red–green colour blindness affects up to 8% of males and 0.5% of females in humans. The ability to see colour also decreases in old age.
About 8 percent of males are red-green colour blind in some way or another, whether it is one colour, a colour combination, or another mutation.
There are two major types of colour blindness: those who have difficulty distinguishing between red and green, and who have difficulty distinguishing between blue and yellow.
Two percent of the male population exhibit severe difficulties distinguishing between red, orange, yellow, and green. Some may not even be aware that their colour perception is in any way different from normal.
Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cells in the eye.
Tetrachromacy is demonstrated among several species of birds, fish, amphibians, reptiles, insects and some mammals. It was the normal condition of most mammals in the past.
One study suggested that 2–3% of the world’s women might have the type of fourth cone whose sensitivity peak is between the standard red and green cones, giving, theoretically, a significant increase in color differentiation.
In humans, preliminary visual processing occurs in the neurons of the retina. It is not known how these nerves would respond to a new color channel, that is, whether they could handle it separately or just combine it in with an existing channel. Visual information leaves the eye by way of the optic nerve; it is not known whether 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.
Everyone’s cones see slightly differently already due to genetic variation, so theoretically the same wavelength of light looks infinitesimally different to every single person. Colour-blind people can’t distinguish between certain colours, not that they’re seeing them wrong.
You can’t diagnose tetrachromacy on a computer screen, because computer screens are made up of combinations of only three different colors of light. It is literally not possible for an LED computer screen to generate the kind of nuance that distinguishes tetrachromats from trichromats.
Computer screen is only capable of generating three wavelengths of light, and all others are projected as mixtures of those three. The sensor of the camera only records three wavelengths of light, and all others are a mixture of those three. Any colour that is not specific wavelength of red, green, or blue is made of a combination of those three by our eyes, brain, camera and screen.
What about prevalence of color blindness by nationality, one might expect the percentage of affected people to be relatively constant in all countries, this is far from the truth. In most Caucasian societies up to 1 in 10 men suffer, however only 1 in 100 Eskimos are colour blind.
Research models have attempted to predict or calculate how colour-blind users perceive colours. These models make a good attempt, but it is not possible to predict what a colour-blind user sees with 100% accuracy.