Shama-il Tirmidhi Chapter 005, Hadith Number 001 (036).

Qatadah R.A. reports: "I asked Anas R.A., 'Did Rasulullah (Sallallahu alaihe wasallam) dye his hair?'" He replied: "The hair of Rasulullah (Sallallahu alaihe wasallam) did not reach such a stage where one would use a dye; however Rasulullah (Sallallahu alihe wasallam) did have a few white hair on his temples, on the other hand Abu Bakr R.A. dyed his hair with Hina and Katm."

Commentary.
Katm is a type of grass, which is used for dying the hair. Some people say that the colour of Katm is black, and if mixed with Hina it becomes red. Others say the colour of Katm is green, and when mixed with Hina it gives a black tint. Mulla Ali Qari says: 'The one that gives more colour is considered. If more Katm is used the dye becomes black and if more Hina is used, it becomes red'. The use of both colours are permissible, but black should not be used. It has been forbidden in the ahaadith to use a totally black dye.