well to be honest, they all brown out. not seveirly.
but its natural. but their yellows get brighter. well mine has.
she's 8 months old and 350+grams and only slighty browned out.
but she's higher yellow than whites and her dorsal is kinda goldish.
they're all different.
look for the lightest cleanest and white ratio you want. and you wont strike out.