Actually, artists acrylic paints do not stick to smooth plastic very well. It will wash off or scrape off easily. And if you thin the paint at all it will not begin to cover. I recommend enamel paint, like the stuff that is used to paint model cars and planes and the such. They are made to use on plastics, glass, metal, and other non-porous surfaces. You can buy a gazillion different colours at your local hobby store, and limited colours in the hardware department.