Sources are quoted by Robert Spencer, a widely read author on the topic of Islam and violent jihdad:
"As I noted here yesterday, the Shafi'i school of Islamic jurisprudence forbids men and women to dye their hair black "except when the intention is jihad...as a show of strength to unbelievers" ('Umdat al-Salik e.4.4). OBL is not a member of the Shafi'i school; the Wahhabis adhere to the Hanbali school, and the Deobandis (the dominant sect among the Taliban) are Hanafis. But this is not just a Shafi'i view: Sheikh Yusuf al-Qaradawi, in his internationally influential The Lawful and the Prohibited in Islam, says essentially the same thing: "Some of the early Muslims, including some sahabah [companions of Muhammad] such as Sa'd ibn Abi Waqqas, 'Uqbah ibn 'Amr, al-Hassan, al-Hussein, Jarir, and others permitted the use of black dye. Some scholars, on the other hand, do not consider the use of black dye as permissible except during time of war, when the enemy might be impressed by the fact that all the soldiers of the Muslim army look young."