Is it permissible to dye hair black with gallnut for darkening purposes, and not for deception or fraud?

There is no harm in dyeing hair for men and women with any color except black. Scholars have differed regarding black dye, between its being disliked (makruh) and forbidden (haram). The most preponderant opinion is that it is forbidden, due to the Prophet's (peace be upon him) saying: "Change this, but avoid black," and his saying: "There will be people in the latter times who will dye their hair black like the crops of pigeons; they will not smell the fragrance of Paradise." The general rule is that a prohibition should be understood as forbidding unless there is an indicator that shifts it to being merely disliked, and there is no such indicator here.