What is permissible and Haram (prohibited)

The first question of Fatwa no. 9407 I have seen some people using a substance to change the color of hair into red or black, or to make curly hair straight. Is it permissible for a Muslim to use such substances? Are the youth and elderly people equal in this regard?

A: All praise be to Allah Alone, and peace and blessings be upon His Messenger, and his family and Companions. It is permissible to change hair color into another color, but to avoid black. Likewise, it is permissible to use cosmetics to soften curly hair. The youth and elderly people are equal in this regard provided that no harm is expected and that the substance used is pure and lawful. However, it is impermissible for both men and women to change the color of their hair into black, as the Prophet (peace be upon him) said: “Change this (gray hair) with something, but avoid black.” May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.