Sometimes I think we hunters think too much.

Every deer first is different. Each is born with a diff. mother and father which gives up their own genetic make up. With that being said some deer are greyer than others, some are browner than others etc. etc. etc.

But as many fur bearing mammals age.. they loose some pigmentation in their hair causing more grey.. or white.. what have you. Similiar to a dog or cat that also reaches older age.

So it's really just a combination of several diff. factors. I would not doubt that diet and health play a role as well.