Shaikh Bin Baz rahimahullah was asked:
Is dyeing the hair lawful or unlawful?
If it is pure black, then it is not allowed for both men or women. If he dyes using yellow or green or other than that, then there is no harm. However, the Prophet sallahu alayhi wa salam prohibited pure black. He said, sallahu alayhi wa salam:
غيروا هذا الشيب، وجنبوه السواد
Change this, but avoid black.
So it is not allowed to use pure black, not for a woman nor a man. If he changes the white hair with other than black (such as) with black mixed with henna or with red or with yellow, then there is no harm”
[Reference removed since it no longer exists]
Faisal Ibn Abdul Qaadir Ibn Hassan