Generally speaking, the silicone rubber has two kinds, tin cure (condensation cure ) silicone and platinum cure (addition cure) silicone rubber. And the part B of both kinds are the hardner.
Usually, the pigment would be added into the hardner. But what we should pay attention to is that the platinum cure silicone rubber is very sensitive, and the heavy mental or the impurities can make it uncured easily. So pls do make sure the pigment is suitable for silicone rubber.
Then, how many to add?
If for the tin cure silicone rubber, add about 2% pigment into the hardner first (100g hardner, 2g pigment.) stir them evenly and then add it into the rubber base.
If for the platinum cure silicone rubber, add about 0.2%~0.5% pigment into the hardner first (100g hardner, 0.2g~0.5g pigment) stir them evenly and then mix it with the silicone rubber base.
Of course, you can adjust the addition of the pigment according to the different shade you want.