Brown paint is already "in" the game... with both a texture, and an entry in the block XML thing (ID, name, description, the whole shabang)... except it isn't in the game because it's set to be disabled. FOR SOME REASON.
Amazing, right? . _.
Because before, people were complaining they had to manufacture brown paint from three constituent colours...
I don't see the necessity to have an extra resource just to manufature brown paint. There are eight minerals, each with their corresponding paint canister. If we should get more (or revive old) paint canisters, then the sensible thing to do is mix them from the existing ones.
In my opinion, it would restrict the possible range of colour choices for any new minerals, if any were to be introduced, if every new mineral would have to have a corresponding paint capsule. Leave it at the current eight "basic" colours, and mix any others (just brown for now) from them. If anything, overhaul the paint recipes to RGB+white and black, mix
all other colours from them, and give the "leftover" minerals some new use.