Colour has always been a puzzle... It's mainly our perception.
There are two kinds of photoreceptors in the human eye - Rods and Cones. I am sure many of you have heard this.
There are about 120 Million of 'rods' - and these are mainly responsible for night vision, because they are highly sensitive to low-intensity light. These are also accountable for peripheral vision. These are totally blind for high intensity light and do not have the colour ability - basically Achromatic vision.
OTOH, the Cones, abot 6 to 7 million, are responsible for the visual acuity (the ability to resolve and to pick up the minor details) and for distinguishing colours.
There are three types of cones:
– Red cones, 64% of the total, also known as L-cones (maximally sensitive to long-wave light).
– Green cones, 32% of the total, also known as M-cones (maximally sensitive to medium-wave light).
– Blue cones, 2 – 7% of the total, also known as S-cones (maximally sensitive to short-wave light).
Normally, the peak of red cone light perception at 565 nm, the peak of green cone at 535 nm and the peak of blue cone at 430 nm.
It's too lengthy to explain everything here but there are pigment-proteins that gives the photoreceptors in the human eye this ability.
Cats and dogs only have blue and green cones. This means they have a much more muted perception of colour than the human eye. Some animals only see one colour. There are Monochromats, Dichromats and Trichromats.
The humans are Trichomats and so are the primates. But some monkeys like the night monkeys are monochromats.
PS: Since some are interested in how animals could see. Just a few more lines.....
Most marine mammals like seals, sea lions and walruses, and cetaceans, such as dolphins and whales are Monochromatic. These Mammals have lost the ability as it's not very helpful for them at sea.
But most fish are trichromatic, with good color vision -except for a few species like skates & sharks.
Cows cannot see the Red colour because they do not have the Red cones.. - (why do matadors wave a red cloth at he bull? - might as well wave any coloured cloth)
Nocturnal birds are colour blind. Moost cold blooded animals can see infra-red. That's how snakes detect their prey.
Another interesting thing is an insects compound-eyes. Dragonflies have one of the highest-quality compound eyes with 30,000 lenses per eye. Birds of prey, do have 1 million sensor cells per square millimeter. The eyes of these birds have two to three times the acuity of human eyes, but the acuity of a human eye is still about 100 times better than that of the best insect compound eye.