It looks pretty good! I don't know that I would call black uncommon (not your words, but you said there aren't many), I've actually owned more blacks then any other color.
You could add sabino to your pinto page. Also don't know that I would say bay roan is rare.
The only other thing, is buckskin and dun are not the same, and a page dedicated to the differences would be good, it's what stumps alot of people. I actually thought you forgot buckskin (which I would say is closer to palomino, then dun), until I saw it on your dun and roan page. I would probably also give roan and dun separate pages, and maybe add a page for uncommon combo colors (like blue roan/grulla, roan palomino, etc.) you could move the pintaloosa there. Ok, so more then more other thing, but also would could do a page that tells the difference between sabino, true roan, and rabicano! Ok, thought of another one, cremello (I know you have one on the palomino page), perlino, and smokey cream!