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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
This action is not included in the prohibited Nams, which means the plucking of eyebrows. Rather, it is just a form of dyeing the hair, which is allowed in the Sharee'ah, unless one dyes his/her hair in black. It is disliked in the Sharee'ah to dye the hair black. It was narrated on the authority of Jabir that the Prophet, sallallaahu ‘alayhi wa sallam, said: "….Change it (the white hair of Abu Quhafah, Abu Bakr's father) with something, but avoid black." [Muslim]
Allaah Knows best.