Are Horses Color Blind?
The answer is yes and no. Horses are not totally colorblind, but they do not see the range of colors we see. In Horses and Horsemanship by Dr. M.E. Ensminger (1977) at the time of that writing most authorities did not think horses could see color at all. But Ensminger sited an experiment done by one of his students that indicated horses did recognize different colors. The student hung two buckets, which were different colors about ten feet apart. Food was placed in one bucket and the other was empty. The horses were allowed to approach from a distance of thirty feet. All the horses tested, “had the ability to recognize colors, although some horses learned faster than others.” In my opinion that didn’t prove they recognized colors. I wonder if their sense of smell might have helped them know which bucket had the oats. But many more tests and experiments over the years since that book was written indicate horses see more than black and white.
We now know that horses do see some colors, but with limitations. There are two factors called photoreceptors that allow animals to see colors: cones and rods. Rods allow them to see in low light conditions and cones are sensitive to color. Horses have just two types of cones, whereas humans have three. Reducing the number to two greatly reduces the number of colors horses see. They have no intermediate hues, but grays and pastels. They see mostly in the yellow, brown, and gray ranges.
There is a good graph that shows the difference between our sight and that of the horse at http://www.mini-horse.org/vision_color.html. An article titled “The Equine Color Debate” by Joanne Meszoly gives more details on how horses see color. You can find the article at http://www.equisearch.com/horses_care/health/anatomy/colorvision_012706/