Ahh.. so my basic points were correct in my original post but I was unaware of an additional "modifying" gene.
Given this information I can see what you are referring to in your posts though, because if you add this At gene to a black based horse and then also add a cream gene you'd get what? A seal brown buckskin?
I'm sure there are still hundreds of other modifying genes out there as well that have yet to be "discovered" as this science of coat color genetics is still technically in it's infancy and there is much more research that needs to be done. It's a fascinating subject though huh?
Side Note: The picture they have on the page you linked in your post of a Seal Brown QH?? To me it looks like a dark bay. Even with such a small pic you can still see the black points and the reddish color to it's coat... but that's just my opinion :)
That's the trick with brown. It is very similar to bay - black points are part of the colour. The difference is that brown has paler areas in the "soft" points - muzzle, eye, flank, elbows, genital area, butt.
This guy shows it in its darkest form:
Then it gets tricky as they get lighter in shade and start to become easily passed of as bay:
And then you get those that I am certain would test brown, that are really really hard to tell. Like this horse. I bet they would test brown: