Rain Lover,
I think the colours other than the 6 I mentioned would not apply to thoroughbreds. EG a thoroughbred cannot be palomino. Albino may be a valid inclusion but I don't think I've ever heard of one. You'd want to back them at the night meetings though, I don't think albinos are real keen on sunshine. ( maybe they'd do ok in Melbourne!!! ).

Le Zagaletta has to be the closest thing I've seen to a white racehorse.