Art History class has got me thinking. Ancient Egyptians used paint, heck,
cavemen used paint. Dwarves could make paint. Dwarves could totally paint things.
The Egyptians used
tempera paints, which if I've got it correct, are pigments mixed with egg yolk, though other biological materials like milk casein or honey can be used instead.
Oil Painting, pigments mixed with oils like linseed or poppyseed oil, started between the 5th and 9th centuries in Afghanistan, but didn't reach Europe until 1500, after the tech cut-off. I'm assuming that the creation of paint is a simple matter of mixing pigment (i.e dimple dye) with binder (i.e. eggs) and that more complicated colors can be achieved by mixing more than one paint together. The metal tubes for storing paint were not invented until the industrial age, so I think paint would have to be mixed on the spot. You would not have paint stockpiles, but rather use the pigment and binder for the painting reaction.
So now that you've got your paints, what are you going to put them on? Those dwarvey Egyptians painted a lot on stone, for example The Sphinx was once painted in brilliant colors and tomb walls were often painted in addition to engraved. But that would be complicated. It would be easier for painters to bring in finished goods or furniture to be painted after production, the same way jewel encrusters do. Cloth (and maybe leather) finished goods should be excluded, since they should be dyed instead, although a Painters' workshop might replace the Dyer's Workshop. (A Pigmentarium?)
But what about just making a painting? Canvas was probably possible before 1400 as it was used in sails, but it was not popular in painting until after 1400. Instead, wood panels were the medium of choice for most of the middle ages. Cue up some panels at your Carpenter, or maybe just use Wood Blocks as they are and then order those dwarves to paint. Out east, paper was also used to make scroll paintings, but we'd have to invent paper first (
1)(
2).
I'm not sure if you can paint ceramics using tempera paints, since glazes seem to be the way of things, except that Greek pottery had art that looks like paintings. If you can, you could make decorative vases or ceramic panels as well.
Back in medieval days, art was often commissioned, so perhaps there could be some mechanic where you could order your dwarves to paint a particular deity or historical figure or a portrait of a fortress subject, or a specific subject like animals, national symbols or geometric shapes. Filling your halls with propaganda posters or abstract art deco or a series of portraits of dead nobles with moving eyes could be amusing.
Another fun little trick could be the ability to paint blocks, so you could build a
big blue gate decorated with wild animals (the real one is actually not pained).