A man is not allowed to dye his hair black, except for the mujahid in the way of Allah or if one has a genuine medical reason. A woman cannot dye her hair pure black either but other colours may be permissible for the pleasure of her husband. Some of the scholars have commented that the reason of the prohibition is that to dye your hair black is a form of deception, i.e. it conceals your age. Wallahu alam.