TEL_dirt.tga and TEL_grass.tga, just to mention two. They should appear in the Restoration Zone, and possibly the gardens on Citadel Station. I also did a test with TEL_fl02.tga, a floor tile that appears in the TSF office, and painted it clear red, but nothing changed. All TEL_-files I paint appear vanilla in-game.
I used PER_-files to change the Peragus textures, and that worked fine.
Also, I have the retail 4-disc version, not the Steam version.