Like Jason said, they have spray paint specifically made for plastics:

http://www.krylon.com/products/categories/plastic/


LFR1100 Actives,QS10HPx2,QS8x2,EP800,M3x4,M3x2 (Wood),M5HPx2 (Wood),AxiomAir,ADA1500-8,ADA1500-7