Not knowing what the material is makes for a challenging 'correct' answer. It is possible that the lense is an aftermarket "smoked" type, where the coloration is intergral to the plastic. IF it is an oil based spray on product, perhaps try using acetone, or mineral spirits (turpentine) - separately, and try a small test area first. Another solvent might be Goof Off. If none of those are successful, you may be looking at a simple light lense replacement. There are plenty of online sources and the cost won't break the bank.