Horses see movement better than anything else. They also see in greyscale, but some colors such as purple and yellow pop out best for them. When I say they see movement better than anything else and their colors are pale and dull, it's like looking underwater and the pool
It's a good question and it's very interesting !!
The difference between the vision of man and that of the horse is twofold. The first is the shape of the pupil which is elongated and horizontal in the horse. It is for example very clearly vertical in the cat, and round in the human. This is what allows the horse to have a panoramic view.
The second is the light receptors. There are several types: sticks and cones. The sticks allow you to capture an image even if there is little light. It is they, for example, that allow us to see in a dimly lit room. But in this case, no color, everything is gray! If one sees color it is thanks to the cones. There are three types that capture red light, others light blue and the last green light. And it is by associating the results of each type of cone that we see all the colors of our spectrum. The particularity of the horse is that the distribution of receptor types is not the same: the horse has no red cones (it has a dichromatic vision) and more sticks than the man.
The fact that he has his eyes on the side of the head allows him to see much better than we on the sides and "behind" him. On the right and left sides the horse sees only one eye while in front of him he sees both eyes. However, the perception of the relief is only possible in the zones of binocular vision.