Praise be to Allah.
Dyeing it with something reddish or yellowish is Sunnah, but dyeing it black is haraam according to the sound opinion, and it was also said that it is makrooh. This ban applies to both men and women, except for the man who is engaged in jihaad (mujaahid). Al-Maawirdi said: “It is not haraam in his case.” In Saheeh Muslim it is reported from Jaabir (may Allaah be pleased with him) that when the Prophet (peace and blessings of Allaah be upon him) saw that the beard of Abu Quhaafah, who was the father of Abu Bakr al-Siddeeq (may Allaah be pleased with him), was so white, he said, “Change this, but avoid black.”
(The above is the words of Imaam al-Nawawi).
Ibn ‘Umar (may Allaah be pleased with him) reported that he saw the Prophet (peace and blessings of Allaah be upon him) dyeing his beard with a yellowish colour. According to another report, he added, “And I would like to do this.”
Imaam Ahmad and Ibn Maajah reported that Wahb said: “We entered upon Umm Salamah (may Allaah be pleased with her) and she brought out some of the hair of the Prophet (peace and blessings of Allaah be upon him) to show us. It was dyed with henna and katam.”
Katam is a plant that is mixed with henna, and it is used to dye the hair and preserve the colour, as is stated in al-Qaamoos. When katam is used alone, not mixed with anything, it produces a reddish-black colour.
Some people have the habit of dyeing their hair pure black at peace time (i.e., when not engaged in jihaad). This is not right according to sharee’ah. There have been cases where prospective brides and their families have been deceived and cheated when a prospective husband has dyed his hair to give the impression of being young and strong when in fact he is old and weak.