It all depends on what you want to do.
Hunters - Brown (any shade really, but meduim brown seems to be popular).
Show Jumpers - Any color.
Jumpers - Any color, but brown is seen most.
Dressage - Black. If only doing Intro or Training then your jumping saddle can be used (with lengthened stirrups), but black all purposes or dressage saddles are perfered.
I think either medium/dark brown or black would look best on your very handsome guy. (:
You can never take a Thoroughbred away from a horse crazy girl.