It's a mystery to me why, since I began a serious exercise program almost exactly two years ago, grey hairs are growing out brown, and my hair is thicker. So testifies my Sicilian barber of 20 years. My body has changed in many desirable ways - less fat, more muscle, cut abdomen (somewhat), better posture, tougher all over - but I do not exercise my hair at all and use no Rogaine, no hair dye.
Could it just be diet? I have tried to up my protein intake and go very easy on carbs except maybe in restaurants. I love mashed taters. Or potato chips which I also love. I tend to avoid vegetables unless in a Thai concoction, or unless cooked in garlic and oil. Or in bacon. Tomato is not a vegetable. I also tend to avoid fruit unless in a banana or strawberry protein smoothie because I do not like sweet things very much and, when I want some, it is never satisfying and makes me feel uncomfortable. I have always preferred savory to sweet. Generally-speaking, my program has lowered my appetite quite a bit so I focus on protein because I am told to. I do like a steak sandwich.
One case is just an anecdote, so it means nothing. I do wonder, though. Hair is mostly genetic but this change is odd.