All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It is permissible to dye part of the hair with other than black and leave the rest black as it is permissible in principle, provided that by doing so, one does not imitate non-Muslim women or dissolute ones as in this case, it is forbidden. The Prophet said: "Whoever imitates a people, he is one of them." [Ahmad and Abu Daawood]. If there is no imitation in doing so, then this is permissible.
Allaah Knows best.