Sayyiduna Abu Rimthah (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him and his family) used to dye his hair with henna and katam, and his hair would reach his shoulders.
Hadith Takhrij«صحيح لغيره، أخرجه الطبراني في الكبير : 22/ 726، والبيھقي في دلائل النبوة : 1/ 238 ، (انظر مسند أحمد ترقيم الرسالة: 17497 ترقیم بيت الأفكار الدولية: 17636»
Brief Explanation
Benefits: ... Katam: It is a herb found in Yemen, which produces a reddish-black color, whereas the color of henna (mehndi) is red. If katam and henna are mixed together, an intermediate color between black and red is produced, which we call (radi Allahu anhuma sallallahu alayhi wa sallam radi Allahu anha rahimahullah radi Allahu anhu radi Allahu anha own).
In summary, it is not permissible to dye the beard and head hair with a color that appears clearly black. For further details, see Hadith number (8201) and the following chapter.