Black, "Safety Yellow", and Forest Green are the standard options that are street-legal. In order for other colors to be used, they're either grandfathered in or an exception was made. Here in RI the signals are all yellow, though some in urban areas are black to be more decorative looking. The backboards (things that bolt around the outside of the signal, sorta like a decorative trim) are all black, but newer installs are required to have reflective yellow tape on the backboards for increased visibility. Our older signals were green in urban areas but they're not used anymore.
Not sure what "faceplates" are but sometimes the signal bodies are yellow and the doors and visors are black. The signals look black from the front and yellow from behind. I never cared much for the mis-matching though. I think forest green looks best on signals but I'd personally keep the original color when you repaint it, whatever it was. If you do not want to keep the original color I suggest a nice medium blue color. I think that would look nice.