accepted and possable are 2 entirly different things, the border collie standards says they should not be mostly white, they simpley dont exept mostly whit BCs, in the real world mostly white BCs are VERY common and there is nothing at all wronge with them, brindle is not accepted either, but they come it that too. this is the same with any breed, just because a a colour is not accepted hardly means the colour does not exist.