Morning headcanon:

Belf hair colour is like everyone else. If your parents had black hair, you’re born with black hair.. but white hair for families isn’t uncommon and also isn’t seen a sign of age or magical transformation (IE: Jaina). 

White hair or whitish hair may have just been something that was passed down and in older elves if they started with another colour and it turned gray, was just unfortunate coincidence. 

Hm what of the blood elf’s death knight hair color? hair color changes/aged upon death?

(What’s awful is that my iTunes switched to a Spice Girls song just as this was posted… )

With Death Knights, that may be about the only time where magic changes appearances? I assume Koltira looked pretty handsome in his life which didn’t seem to transfer over too much to his new unlife.. but I digress. 

I suppose a Jaina-esque situation would here whereas magical properties caused a physical change. But that also happened in multiple races.. so I don’t think that instance is limited to belves. (Unless he died with white hair.)

It outright states that Arthas’ hair color was changed due to the influence of Frostmourne. I would assume that something similar would happen with other Death Knights. (My working theory is that the glowing eyes and pale skin are more than enough of a reminder of what he is, so Tev colors his hair.)