Praise be to Allah.
It is permissible for a woman to dye her hair any colour other than black, so long as that is not imitating kaafir women, such as if kaafir women or some kaafir women have a special way of dyeing or cutting their hair, and that is exclusive to them; it is not permissible to imitate them in that case.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: Is it permissible to dye parts of the hair such as the ends or the top only?
He replied: If the hair is to be dyed black, the Prophet (peace and blessings of Allaah be upon him) forbade that when he told someone to change his white hair but to avoid black. He said: “Change this white hair but avoid black.” Saheeh Muslim (5476). A stern warning was also narrated for the one who does that, which is when the Prophet (peace and blessings of Allaah be upon him) said: “There will be people at the end of time who will dye their hair black like the crops of pigeons; they will not even smell the fragrance of Paradise.” Narrated by Abu Dawood (4212) and al-Nasaa’i (8/138); classed as saheeh by al-Albaani in Saheeh al-Jaami’ (8153). This indicates that it is haraam to change the hair colour to black. As for changing it to 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.” Narrated by Abu Dawood (4031) and classed as saheeh by al-Albaani in Irwa’ al-Ghaleel (5/109). End quote from Majmoo’ Fataawa wa Rasaa’il Ibn ‘Uthaymeen (11/120).
And Allaah knows best.