I wasn't going to join in on this discussion because I've sorta got skin in the game; I'm not far off 51 and have long hair (and plenty of it, happily!). I find even the thought of dyeing my hair to be utterly hilarious. I think it looks great going grey naturally. Opinions, opinions, of course but I think James and John have looked, uh, sub-optimal for a while: 1) dyeing your hair jet black, particularly as an ageing male, almost never works (skin pallor), 2) ironing your hair almost always takes it to another level of hilarity on top of that, 3) ...and when combined with a dyed beard, too? Yeah, not for me. But that's the thing, really. I hope they do it because it makes them happy, not for any other reason.
Look at Kirk Hammett or Tom Araya. Great grey/greying rock hair. Brilliant stuff. Own it!