Is it permissible to straighten hair with herbs that change its color to dark black due to the presence of some white hairs, for the purpose of adorning oneself for the husband, noting that the questioner wears a niqab?

It is permissible to use natural herbs to straighten or soften hair, so long as they are not harmful. As for dyeing hair black, it is not permissible for women or men, young or old, due to the prohibition of the Prophet, peace and blessings be upon him, and the severe warning against those who do it. If these aforementioned herbs give a dark black color, then it is not permissible to use them for someone whose hair has turned gray.