Sayyiduna Abu Rimthah (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him) used to dye his hair with henna and katam, and his hair would reach his shoulders.
Hadith Referenceالفتح الربانی / 11141
Hadith Gradingمحدثین:صحیح
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; it produces a reddish-black color, whereas the color of henna (mehndi) is red. If katam and henna are mixed, an intermediate color between black and red is produced, which we call (radi Allahu anhuma, sallallahu alayhi wa sallam, rahimahullah, radi Allahu anhu, brown).
In summary, it is not permissible to dye the beard and head hair with a color that appears clearly black. For further clarification, see the next chapter.