James is about 14, black, and has a definite ginger tinge, particularly on his chest. Christopher is 12, also black, and shows no sign of a colour change - and he spends more time sunbathing! They both eat the same food, wet and dry.
While it might indicate a nutritional deficiency, it's much more likely to be down to genetics and age. A bit like humans going grey
. Provided Ben doesn't show any other signs of a problem, such as loss of weight, I wouldn't worry about it although if you remain concerned you should talk to your vet.