Praise be to Allah.
If the hair is dyed black, the Prophet (peace and blessings of Allaah be upon him) forbade that, when he ordered changing white hair but avoiding black. He said: “Change this white hair but avoid black.”
There is also a stern warning against doing that, which indicates that it is haraam to change white hair by dyeing it black. As for changing it by dyeing it other colours, the basic principle is that it is permissible, unless it is done in the manner of kaafir women or immoral women, in which case it is haraam for that reason, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever imitates a people is one of them.” End quote.