Slapped it together and did some testing. It's easy beyond reason... Smelter tags accept "any stone" and turn it into "red paint bars." It's designated as a metal for testing purposes, but it can't be used for anything but making vertical/floor bars (it uses the [BRITTLE] tag that pig iron has).
Testing block versions now.
Works fine.
I made it a metal because I don't know how to make it into a stone that won't be actually used outside of the smelter reactions. In my experience, putting the [DEEP] tag on a metal ensures that nobody will use that metal, even if they can theoretically produce it, aside from the fortress. (I once created [DEEP] Magmatite, a metal with all the properties of bauxite that could be refined at a punishing ratio from iron bars and that could be used to make mechanisms, a la HFS).
You could probably melt the paint blocks, at which point you'd wind up with paint bars which would be utterly useless for anything aside from making floor/vertical bars...
Also, you have to start out by making "primed blocks," which basically was my way of ensuring that my smelter wouldn't always be clogged by fifteen different "MAKE PAINT BLOCKS" reactions. Primed blocks are made from any stone and paint blocks are made from primed blocks...
I'm going to finish this one up some other time, once I feel like adding enough colors, and then if anyone cares I'll put it up for you.