While for ages it was commonly believed that all animals were completely colour-blind, research has shown that many animals – including dogs, cattle, pigs, goats, sheep, deer, and horses – can see some colour.
In order to see in colour, an animal must have at least two different classes of receptors, which are light-sensitive cone cells in the retina (back) of the eye. Humans and primates typically have three different types of cone cells in the retina, called trichromatic (three colour) vision and can see four basic colours – red, green, blue, and yellow – as well as many intermediate hues. A horse’s eye contains two classes of cone cell, and thus has dichromatic (two colour) vision. One cone class absorbs blue light (short wavelengths), and the other absorbs green-to-red colours (middle to long wavelengths), although horses do not see the green-to-red colours the same way as people, as their cones are different than human eye cones. There are also dichromatic humans who suffer from either red, green, or blue colour blindness.
A 2000 study by veterinary researchers in Wisconsin showed that horses, with their dichromatic vision, cannot discriminate red in the red/green region of the spectrum, and are also slightly less sensitive to red light. A newer study published in 2007 by Hanggi, Ingersoll, and Waggoner confirmed that horses are dichromats with colour vision capabilities similar to those of humans with red-green colour deficiencies. These researchers used a pseudoisochromatic plate test (different coloured dotted circles set against backgrounds of varying dots), highly effective in testing colour vision in humans, but never previously used in animals.
With their reduced colour perception and slight inability to see details as clearly as humans, horse vision might be roughly equated to a slightly near-sighted colour-blind human; however, a horse’s night vision is far superior to humans.