I suggest SEM paint, which is a dye-like paint meant for hard-plastic automotive interior parts. I recall it in a variety of goldish tones, and you can have it mixed to order for airbrush use if you want something custom, or simply buy the offf-the-shelf spray colours. It will last a very long time and doesn't scratch off (at least not without effort) as it forms a quasi-molecular bond to the plastic. It works on hard, semi-hard and soft plastics: I've used it on buttons, case shells for games and consoles, vinyl and rubber...
You can also use oil/candle dye on very soft plastics, which you can pick up at craft stores. This can be handy for colour-coding wires without buying giant reels of multi-colour-cased stranded or solid-core copper wire, which costs a fortune now-a-days. -GG