The basic principle concerning things other than acts of worship is that they are permissible. Based on this it is permissible for a woman to dye her hair if she wishes, unless it is black dye that is used to conceal grey hairs, which is not permitted because the Prophet (peace and blessings of Allaah be upon him) enjoined changing grey hair but he said “avoid black”; or if this dye is something that is used only by kaafir women such that if this woman was seen, people would say that she is a kaafir woman, because no one uses this dye except kaafir women. In that case, it is haraam for a woman to dye her hair with it, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever imitates a people is one of them.” If the dye is free from these two elements, i.e., black to cover grey hairs and dye that is used only by kaafir women, then the basic principle is that it is permissible, so the woman may dye her hair if she wishes. End quote.
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him).