It is not allowed to dye your hair black - regardless whether it is your beard or the hair on your head. Certain sayings of the Messenger of Allaah (sallallaahu alaihi wa sallam) indicate that dyeing ones hair to red, yellow, or with a mixture of henna and Katam. He (sallallaahu alaihi wa sallam) clearly said: Change the whiteness in the hair, but stay away from black.  Recorded by Muslim in his Sahih, and Abu-Dawud, An-Nasai, and Ibn Majah - in a Hadith related by Jabir bin Abdullah, may Allaah be pleased with him, the Messenger of Allaah (sallallaahu alaihi wa sallam) said: Indeed, neither the Jews nor the Christians dye their hair, so differ from them.  And Allaah is the Granter of success.
 Muslim no. 210, Abu Dawud no. 4204 Ibn Majah no. 3624. An-Nasai no 2103  Muslim no. 2103