I'm painting a large family room with an 18 foot sloped ceiling. The room has about a dozen HVAC registers - most are just above the base molding, but a few are placed in the wall about 10 feet off the ground. The room also has in-wall speakers about 36 inches off the ground. Is there a general rule as to what colors to paints HVAC registers and speaker grills? I seem to notice the ones high on the wall being painted the wall color and the lower ones being painted the trim color. I would appreciate any suggestions.