Qatadah RA reports:
"I asked Anas RA: 'Did Rasulullah (SAW) dye his hair?'
'The hair of Rasulullah (SAW) did not reach such a stage where one would use a dye. However Rasulullah (SAW) did have a few white hair on his temples. On the other hand, Abu Bakr RA dyed his hair with Henna and Katm.' "
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 Henna it becomes red. Others say the colour of Katm is green, and when mixed with Henna it gives a black tint.
The scholar - Mulla Ali Qari RA says:
"The one that gives more colour is considered. If more Katm is used the dye becomes black and if more Henna is used, it becomes red."
The use of both colours are permissible, but black should not be used. The use of totally black dye has been clearly made forbidden in Ahadith.