you cant im afraid, they are white because the pigmentation in the root is dead. i have this smae problem with my black mare, sorry to be the bearer of bad news.
Provided the original cause of the scar is totally healed I would try a permanent human hair dye. Do a small patch test first though, just to make sure the horse is not allergic to it.
This works brilliantly on black and dark brown but I've no experience of trying it on other colours. Not a cure I know but it does work if its purely for cosmetic reasons.