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You can paint the cover to the outlets and light switches and the vent and return covers. However, the switches and the outlets themselves are trickier. If you don't have a lot of them, you could replace them with black switches and outlets. I was in the same situation with a brown room and I left the switches and outlets white. It just didn't bother me enough to change them.

For the air vents and returns, instead of painting them red, you might want to spray paint them with black enamel. Spray enamel is less likely to drip and clump in corners.

Lurton makes switch plates in a very wide variety of colors. Check and see if they have what you need.

You can also buy new vent covers in various colors of metal. I don't remember the place I used but Google architectural grillwork or vent covers. I don't know I'd metal will look better but it is woth checking out. They are not expensive.