this is mainly normal mate.....
Its a sign of stress and could be caused by lots of things...
ie. two male's will change colour if they are close to each other and ready for a scrap..
also feeding time is a big colour changer especially were female's are concerned..
This is perfectly normal.......
The only time to worry with a BN's colour is when they go all one colour like grey...and stay that colour , then you can worry....
With albino Bn's they will tend to go from yellow/white to a pinkish reddish colour with a dark red patch on thier fore head when stressed........
Hope this help's mate......