Why is my sister's hamster...?

changing color? For the last two weeks or so, it's been sort of hiding in it's cage in a mountain of bedding. Today is came out and it's snowy white! Before it went into hiding, it was a really dark grey color. It's a russian dwarf. She bought it in october or so when it was a baby, it's a male. I bought mine at the same time, female. She was a baby when I bought her too. Mine did the same thing, hiding for about two weeks. However, mine didn't change color at all. She's still a grey color. I thought maybe my sister's hamster's fur changed color because of winter or something? I don't know if hamster's fur can do that. I also read that some hamster's hibernate. Is that what our hamsters where doing for two weeks? Because I literally didn't see that out at all. I would constantly check to see if my hamster was still alive because I had no idea why I didn't see her! After two weeks, they both came back out. They still sleep of course, but only during the day now. Two weeks seems like a short period for hibernation, then again I'm not hamster expert. :)