It is okay for a woman to dye her hair in any color except if she’s a very old woman. This is because dyeing her hair is considered to be a deceit.
Sheikh Husnain Makhloof (may Allaah have mercy on him) says: It is permissible for a woman to dye her hair into black with the permission of her husband, and a sign of consenting is for him to approve and agree with her.
Imam Nawawi said in his commentary on Sahih Muslim:
The reddening of face with facial powder, dyeing hair with black color, and beautification of the hands are permissible. If she does not have a husband or she does it without his permission, then it is forbidden but if he gives the permission, it is permissible.
Doctor Qardawi says:
Dyeing of hair into black is permissible for a woman that is not extremely old or an extremely old man because the Prophet peace be upon him, when he came to Abu Qahafa (who is Abubakar’s father) and his hair was all white, so he (peace be upon him) said, change it and avoid black. As a result of this some scholars said dyeing black is not permissible, some others said it is permissible except in the case of Abu Qahafa. It was also narrated that Muslims used to dye their hair black during war to appear as youth in front of the enemies.
Some also said that if the face is young, there’s nothing wrong with it but the old woman should not dye her hair.