I’m not sure how (or if) any of this would be possible in the current game or in UE, but here goes…
How about the possibility of two-tone paint on cars, either have a fixed area for a second colour, or being able to trace on the area ourselves?
The other thing, which I’m sure has been suggested before, is decals. Not necessarily numbers or anything, but stripes on the hood/roof, along the side, etc. and have the same colour wheel as the paint. Maybe have the two-tone implemented as a decal overlayed on the paint, if that’s simpler. Also different stylized decals could be nice with this, such as these:
(Yes, American land barges of the 70s and 80s just can’t be the same without faux wood panelling )