Our school bus fleet I work on has about 30 lens per bus, times 170 buses, so a lot of plastic lens! When they fade, the color goes away and no amount of polishing will help. You can make them shinny, but the color will not come back. We install new, but I would bet, the translucent paint on the inside would make a pretty good improvement. Painting the outside would likely work also, just won't be as robust.
