I thought that this experiment was cool: Colour vision in horses - do horses see colour? | Horsetalk - equestrian feature articles
Their conclusion: "The answer is yes.
- They can definitely see the difference between blue and grey, between yellow and grey and between green and grey.
- It appears that yellow and the green we used look similar, although we need to study this further.
- They can tell red from grey to some extent but they find this much more difficult than the other discriminations."
From my own observations I think that my horse can see hot pink, or at least differentiate between it and other colors. She will spot any bright pink object from across the room and insist that I let her sniff it. She will do this every day until the new object has become dirty and turned brown!