Well, there are two things you can do. The first [which is what I did, to clean the coloured plastic] is to bend up the clips which go all around the taillight, and eventually separate the top platic and SSE grille. You then can remove the the coloured plastic for cleaning.
The other thing you can do, which is very risky, is to separate the top plastic from the SSE grille. I have never attempted this, however I know it is possible. You will need a hair drier, a flat scraping tool perhaps used for paint, and a lot of time. You then take the hair drier, and warm up the top plastic [the area where you can see the top of the tailight from the trunk, when installed], and take the scraper and SLOWLY keep prying at it. I don't think the taillight is glued anywhere other than there, and at the bottom, however it is possible that it is glued also on the SSE grille fins. You will have to complete the first step before you can do this.. or at least it makes things much easier.
Like I said before, this takes a LOT of time and patience.
If anyone has done this, please chime in, I am afraid I did not give enough information.
Once you get the plastic removed, it'* all a matter of cleaning it up. If all else fails, you will probably need a small buffing tool, with some plastic polish, or car paint polish that won't harm the plastic [chances are, that is all of the products available].
Hope that helps!