Originally Posted by Speed Racer
Don't ride the head
Couldn't have said it much better.
Good personality/attitude, physically sound and capable to doing the job the way you need it done.
Color, expressions, etc.... are just things that might make a difference if all else was equal when selecting the horse.
But it's seldom that all else is equal. My youngest horse made my short list of 5 out of the many horses available to select from. Even then all things weren't equal and it was her chest (a tad larger than the other 4....yes, I can be that nick picky) with it's potential for building better lung capacity than the others that won her out. So even there, with 5 horses that measured up for what I wanted all things weren't equal with them.