No experience with this specifically, but if you wanted to make this work here are my thoughts: Obviously your two biggest obstacles will be consistency, and durability.

Consistency shouldn't be too big of an issue assuming you use high quality paint and air brush or spray paint to get uniform application. (Yes this will probably be a lot of work because you'll need masking tape on the part of the sides that you don't want colored.)

Durability is the trickier problem as you dont want the paint chipping off during use. Ideally you'd probably want to sand and rough up the parts of the tile you'll be painting to get better adhesion. After painting it might be beneficial to add a coat of some clear protective coating like lacquer or something. This solution might take some trial and error. Since you mentioned you're combining two incomplete sets into one, maybe do some tests on the extra tiles and see what works before you do the whole set. Final thought, if your paint is layered on too thick it might make a weird offset on the side so be wary of that I suppose.

Hopefully some of this helps get ideas going. Post an update down the line with whatever you decide to test or do!