Most products will get a bit dirty if your applicator contacts any rubber trim, and sometimes plastic trim as well. Cleaners and polishes in particular will have this problem.
Pure waxes like Paste Glaz and Nattys have never given me problems when applying them directly to plastic. But I have several applicators that have black on the outside edges, because I got a little bit of product on the rubberized trim on the edges of the windshield/windows.