Is it true that Uthman ibn Affan and Ali ibn al-Husayn used to dye their hair black?

It has not been reported that Uthman ibn Affan, may Allah be pleased with him, dyed his hair black or any other color. However, it has been reported that Ali ibn al-Husayn and a number of Companions and Tabi'un, among them al-Hasan and al-Husayn, the sons of Ali, Sa'd ibn Abi Waqqas, Uqbah ibn Amir, and Abdullah ibn Amr, used to dye their hair black. The scholars have differed on the ruling regarding dyeing hair black.