Pretty simple, you make pigments out of various things like dimple dye or limonite or hematite or charcoal or orpiment or cinnabar or malachite and so on, and you mix them with, say, rock nut oil or linseed oil, and you get paint.

You then have a designation tool similar to smoothing or engraving for painting floors and walls new colors.

Possibly also furniture and such too.

As opposed to building with limonite, which would give you 4 blocks of coverage, one boulder of limonite (AKA "yellow ochre") ground into a pigment and spread out over a thin layer of paint might be able to cover like 50 tiles or something.

Perhaps it only has modest room value and overwrites other room values of surfaces (since you can't see that the wall is made out of XYZ anymore if it's painted over)