All right I discovered this on accident... I did the age cheat so that I was 18 again, and I still had grey hair!! Well I was very bitter for a while; till I decided that the women in town liked my silver hair Hero. Mind You I was 100% good. Now I went to the temple of skorm to get the bow and after the donations I was neutral well there was no immediate physical change till I warped to the guild and my hair was BLACK!! I am also a will user so I believe it to be that will user does nothing to the hair. The more good the grayer you get and more bad the more and less you have.
Fable rocks
If You Want Black Hair Again!!!!
There is two missions you can do, Maze's Request when you go to find the archelogist is one and Knothole Glades Ransom Victim. When you do either of the quests make sure you have alot of Gold atleast 200,000 gold pieces. (you can get that by selling and buying potions over and over again if you already didn't know that.) So anyways go to the temple of avo during the mission, and donate the money to the temple. ( I always donate 50,000 a pop.) And make sure you are about half way to the dark side to get the age reduction, the sentinus and the paladin name. When you get the Age Reduction do a hero save and do it again, and again till your 18!! 