re: Switching on hidden genes? Chromosomal editing.
This is called atavism in biology, and even random switch-ons might result in successful adaptations given enough time.
This would be particularly true of something that is varied so easily, like fur color. The color of a rabbit varies widely even in normal variations. How long would it take for white to become dominant in a snowy environment? There's not that much difference between polar bears and brown bears, and I suspect polar bears who spent a few tens of thousands of years off the ice would wind up brown again.