I noticed a alien head that had a white pearl effect to it. I think they still make an acrlic white pearl paint by createx. So you could use an airbrush, or touch up gun to apply the paint. The trick is spraying it on even, but if you screw up you could wipe it off with alcohol. Since its non-solvent based, it shouldn't hurt the clear plastic at all. You will still need to add some transparent black, to the front, back, and sides, from what I recall. Then spray it with a clear coat, to protect the paint. Tamayia ( I know I didn't spell that right! ) has a real nice smoked black paint, that looks real good as well. No matter what you should spray some test panels, to make sure what your using will work. Also with it being transparent, you have the luxury of painting from the inside, so it will look nice and shiny on the outside.
I think the best paint for the job, would be automotive paints, but not everyone has access for using that paint. It breaks up, and lays out so much easier than acrlic's. Then again, that's my opinion!