Try the clear-color spray cans for polycarbonates from Tamiya. I believe they come in the primary red/blue/green... but check out 'smoke' grey. - make sure you're buying the CLEAR colors though!
You can then decide to tint it as dark or light as you like with progressive coats when the previous one dries. Don't do one heavy coat or the paint will run & spoil your finish!
Hope this helps.