Okay, I went back through all my old stuff and found some new. Horses have 2 sets of cones which allows them to see only certain colors unlike people who have 3 which allow us to see all the colors. Depending on which study you read depends on exactly what colors they see.

This is from the University of Wisconsin

Color Vision: To examine color vision, very specific wavelengths of light (specific colors) were projected into the eye and the electrical response of the retina measured. Using this technique it was shown that the horse has two types of cones (humans have 3) and therefore has more limited color vision than humans. Horses actually lack the ability to see red, and the color wheels show the differences between what colors horses can see and what we can see.


This is from http://www.omafra.gov.on.ca/english/...rld.htm#colour

Colour Vision
There is a common misconception that most domesticated animals cannot see colour. This is not true, although their colour vision is not as good as ours. The ability to see colour depends on having several different kinds of cone receptors in the retina. Most humans have three different kinds of cones that allow us to see the range of colours that we do. Colour-deficient humans are typically missing one of these cones and, although they can still see colour, they will confuse, say, reds and greens and browns. Horses only have two cone types, so they probably see the world in the same way that a red-green colour-deficient human would. They can easily discriminate red or blue from grey, but find greens and yellows much more difficult to distinguish. Greens and yellows probably look more or less the same colour, but they will appear to be lighter or darker than each other. The horse's world will not be as colourful as ours, but it is not black and white. Colour is important because it allows us to distinguish objects from the background. Poor colour vision reduces this ability. So, a sitting rabbit or bird may be seen readily by the rider, but may remain invisible to a horse until it moves, causing the horse to spook.



Hope this helps. So it looks like they can tell red from blues but they can't really tell if the grass is greener on the other side of the fence. This might explain why my mare years ago used to spook at my red rain coat?