Colour changes in rats?

I got my very first rat not too long ago, just this Christmas past. Her name is Doris (as lots of you already know), and she is my favourite little pumpkin.

I spend a lot of time with Doris and have looked at her up close many times since I have known her. When I first got her, she was completely black. I have pictures of her all over this site from Christmas up to a few months ago, and she is as black as night.

But recently, her colours have changed drastically. She has gone from being pitch black to being hooded, within about a month. I noticed her gradually getting lighter, and now she is two completely different colours. Her upper half and a streak down her back are still blackish, but her bottom half, her underbelly, and everything else is a very light brown, similar to a light agouti.

Summer is coming now, and I'm assuming it's a seasonal thing. I've just never read anywhere that rats (at least Rattus norvegicus) change their colours seasonally.

Is it a common thing?