I vote for painting.
If you do paint, use a latex-based paint. That way you could always scrape it off if you wanted to change it. Just make sure the surface is really clean first so it sticks good. Also, put a piece of masking tape along the frame on the top and bottom or the paint could creep up under it.
I originally did mine light blue/aqua but I just recently scraped it all off and redid it in dark blue. I think it gives a better contrast to the bright fish/corals.