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.
Plucking any part of the eyebrows, if they are normal, is not permissible, even if they are large or contain gray hair, because this falls under Nams (plucking of the eyebrows), which is forbidden according to an injunction from the Prophet, sallallaahu ‘alayhi wa sallam. However, if it constitutes a deformity to the person's face, then it is allowed to remove the extra hair. As for gray hair, it suffices to dye it with something other than black.
Allaah Knows best.