Painting the "ceiling" part will probably do no good -- it doesn't really catch any light that could be reflected. If folks are painting anything, it would be the plastic where the clear light channels sit.

However, on my '84 300td, I didn't want the hassle of prepping, masking, and painting, so I lined the bulb housing and plastic around the channels with metallic duct tape. The tape conforms well and is thin enough not to affect the fit of anything. The lighting is now about four-to-five times as bright as it was before (and that's with amber bulbs replacing the clear, which in itself reduces light output).