Hair doesn't grow in or go grey evenly. On my head, new strands often come in grey, so that when I get a haircut, it reverts to a darker shade of "iron grey". It's even odder with my beard. Up until a couple of months ago I shaved to leave behind sideburns and moustache, and they've remained mostly solid black. But when I let the beard grow, the new growth came in solid grey, so my facial hair has a distinct pattern to it. You don't usually think of humans having mottled coats like a sheepdog, but I sure do.

So, my point is, there can be perfectly natural reasons for grey to seem to come and go on a particular person. I know, unsatisfying to a partisan looking for an attack, but there you go, reality can be so damn inconvenient for partisans.