Is it permissible for someone who is not old (in his thirties) to dye his hair black? I am aware of the Hadith that orders Muslims to change the color of their hair when it is white, but I heard that the phrase “but not black” is part of the Hadith. I also heard other narrations and opinions of scholars that allow dying the hair with black.
I have another question about a white substance that regains the natural color of the person’s hair with repeated use without having any dyes in it. Is it permissible to use it?

Scholars are of three opinions regarding dying the hair black. One opinion allows it, the other dislikes it, and the third prohibits it. Each group has proofs supporting its view. One Hadith is the one you mentioned narrated by Abu Quhafah (may Allah be pleased with him) where the Prophet (peace be upon him) ordered grey hair to be changed and said, “but not black”. Most scholars consider this Hadith a proof of undesirability and not prohibition. This is the opinion of Imam Malik, Abu Hanifah, Imam Ahmad as stated in Al-Mughni book, and most of the Shafi`i scholars. Only Imam Al-Nawawi supported the prohibition. Another scholar who believed in the permissibility is the Hanafi scholar Abu Yusuf who gave some explanations to Risalat ibn Abu Zaid with certain restrictions. A group of the Companions dyed their hair with black such as `Uthman, Al-Hasan, and Al-Husain (may Allah be pleased with them all). Among the Followers who dyed their hair black are Ibn Sirin and Abu Burdah. Ibn Al-Jawzi compiled a paper in this regard entitled Husn Al-Khitab fy Al-Shaib wal-Khidab. Generally, it is permissible unless a man is deceiving a woman into marrying him. But if he is already married and his wife knows the color of his hair, there is no harm in dying his grey hair to look nice for her, especially in the case of the inquirer, who is still young. Dying the hair black is not desirable for old men. That is why some scholars thought it is prohibited, considering the case of Abu Quhafah (may Allah be pleased with him). Please refer to Fath Al-Bari by Al-Hafizh Ibn Hajar and other books of Sunnah and Fiqh. Finally, Allah knows best!