Agree with what others have said. If the grunge is on the lens it can be sanded/buffed off. If it's in the lens you'll need a replacement. I've used 1500/2000 grit wet paper for removing sun discoloration and then polished with "Blue Magic" polish on a cotton ball and had much success on colored lenses. Finished it off with a plastic polish I had for my cycle windscreen as others have suggested. My method described above is likely more appropriate for more heavily scratched/discolored plastic compared to other methods described using less gritty substances for lighltly damaged/discolored plastic.
Do not use windex, I think the isopropanol in it crazes the plastic.
