The kids in my 4-H club usually use tempera paint or spray-on hair color, like you can buy for Halloween costumes and stuff. We've never had a horse react to either of these kinds of paint, and it tends to wash off easily.
I would really advise against using Krylon or something like that on him... either non-toxic tempera or something designed to go on hair.
In the future, Twinkle has tons of paint designed especially for horses.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown