Some examples of how to paint or render plastics. In short, I'd start with a matte surface, make sure you can see through it, if the plastic is translucent or transparent, and then run a final pass to make the specular highlights a little stronger, based on how shiny you want your plastic. All these features can be adjusted to taste, it just depends on the plastic you are going for. The second page attached runs a range of different optical effects and even hue shifting the color as the light intensifies. The more dramatic the hue shifting, the more metallic/ pearl painted the plastic will feel. Essentially, at this point, it's starting to become a mixed material.
