Normally I just make batch of silicone the right colour so it comes out correct, but i have a full silicone torso in the works which needs things like buttons painted. The silicone I use is far too thick and gloopy to use as a paint, so I wondered how anyone else would go about doing it?
I thin the silcone with naptha (sold here as Shellite, also as lighter fluid) to paint smaller details. For heads (like my Poe avatar), I paint the dark hair into the mould first, then the skin tone, then pour silicone in to fill it. So for details like buttons I would probably go the same way, paint them into the mould first. I do paint onto the surface of the cast as well, but too much pigment can inhibit the cure, which means I might need several coats to cover it well - and that could glob up the details and surface texture. So it's better to get a base colour first by painting into the mould, and only add light shading onto the surface of the puppet.
I've used the naptha with caulking silicone, as well as with soft 2-part platinum cure silicones like Platsil Gel-10 and Smoot-On Ecoflex 00-30.
Even with the thinner the caulking silicone was tricky to work with, but in it's original state, like very thick axle grease, it was impossible to paint with.
Thanks Nick! I will try all of this and get back to you!