Normally, these covers are not painted. The slick plastic covers don't accept paint well. The key to painting them is a light sanding with 180 grit sandpaper and using a good primer designed for plastic, XIM in a spray can works really good. The commercial metal types are much better and can be painted after priming.
If you want them painted use the same color as the walls. This will help blend them into the wall. The same sheen is fine. If your walls are flat then paint them flat.