I have used bleach to lighten antler before with great success. I would be tempted to use a 'Dylon' fabric die. I think you could expect varying success - parts of the antler would be more dense than others and parts would be more porous. There are 'natural' dies for brown, red, yellow, blue and green, but none, I think, for grey. There are wood stains, but I have not seen any grey coloured ones. Good Luck.

Regards

Chas