Originally Posted by traction fan
tiger; Craft paints work quite well on plastic. Being water-based, rather than solvent-based, they can't harm plastic.
Thanks. I ended up at a hobby place called Hobby Lobby; apparently a national chain. Wandering around the store, it occurred to me that I would know what qualified as "craft paint", until I got to the far corner of the store and found a huge display called: "craft paint".
Brand name was "Folk Art"; I bought four different bottles, the two I liked best when testing at home was #424 Light Grey for concrete, and #938 Licorice for asphalt. YMMV. Really cheap, too; on sale, I got 4 bottles for under $5 and I picked up a pack of "craft brushes" to be safe.
It did rather well on the Walther's Street System plastics. When I transitioned to the buildings' mounting "plates" made of scrap Lexan, a little more trouble getting a smooth coverage. One issue is this stuff is really thick and not easy to spread evenly, especially on a surface with little "tooth". Then... Duh! I found small containers and made watered-down versions, much better but of course being thin is takes a while to build up enough opacity. I'm several coats in, but I think the results should be fine.
Thanks, everyone, for the input!