"Winter Whites" are called so because, surprise, their fur goes white during the winter! It doesn't always happen in captivity, nor completely. This is Grenville, aged about 3 months, in a "before and after" comparison -- not altogether white, but definitely whiter.

Perhaps that's why he's such a recluse these days -- in his little hamster heart, he knows he shouldn't be outside unless it's on something white!