In britain we have a palomino registery, they will accept any breed as long as they are palomino in colour, not sure I see the point as they are very few 'colour shows' here. I like the idea that if they a registered they are easy allowed to enter shows, everything is proven etc, etc. but I see there being more of a point in them being registered for breed like AHS and APHA and also BSJA and BD (british sj assoc. And British dressage) so they can compete.
I agree with clouds though all horses of a particular breed like AQHA should be accepted into the registery, if they are pure or part bred (qh say) then I really don't see why not. It would make things difficult if they are registered without a colour and they are used for breeding... :/