I own horses and have been around them for years... this horse is fine. It doesn't appear to be "skittish" at all. People "restrain" their horses all of the time (grooming, trailering, etc.) with no problems.
Horse owners know that you have to use LOTS of chemicals to maintain their health, such as supplements, dewormers, and, oh no, daily fly sprays that are LOADED with chemicals and probably far more toxic than that paint.
Horses are EXPENSIVE, so no one in their right mind is going to put such a delicate creature in harms way. I'm sure that they did a patch test on the horse (sound familiar to anyone that has dyed their own hair??), and promptly washed it all out of their hair after the shoot.
C'mon people, use some common sense. If you don't know what you are talking about, keep your mouth (hands?) shut.