Well, it's hard to say whether they are colorblind or not. Some say yes, but scientists have yet to prove it either way.
Also, horses have near 360 degrees of vision due to the placement of their eyes. The placement is also a large indication of their status as a prey animal.
In addition, the placement gives horses a very poor depth perception, and this is why some horses are often hesitant to enter even a puddle of water. They have no way to tell how deep it is and they are afraid it may be bottomless.
This is a great site: Equine Eyesight