Pudding hamster about 2 months old recently his back is turning black i wonder what's happening?

because to get a pudding hamster u must breed a (best case scenario) FEMALE CAMPBELL to a MALE WINTE WHITE, which allows the babies to safely pass through the mum during delivery as the campbell baby heads are bigger than winter whites and if winter white females were used the babies would get stuck...... as for why? cuz u have a cross, that uses genes from both types of hamsters, and winters change colour, and the campbell has black on its back, so its either temporary or it maybe permanent one reason to be careful when breeding them to know the mum and dads history.... what genes they carry influence the offsprings they create