Okay, forgive the idiot here but I've had a bit of a lightbulb on palominos. Cream is responsible for palomino, right. It's also responsible for perlino, cremello, smoky cream & smoky black (on its own). Everything I know about cream says that it affects hair uniformly depending on the base - red will be lightened before black, and some black can take two cream to have a noticable difference.
Ugh, clumsy explanation, but I'm sure we all know what cream does. But, here's the kicker - if its supposed to affect hair at the same rate, why do palominos have white manes instead of them being the same colour as their body? Both parts are red, and it does affect according to expectations on bay for buckskin - the red gets lightened more than the black.