Check out this video Adam Christianson at MacCast found for us. It shows how to remove the yellowing that occurs on vintage computer cases (like a Mac Plus from 1989). If you have a vintage Mac, you know that yellow. The cases turn a specific shade of yellow-brown, and after a while you forget it was gray, or even platinum, when it shipped. YouTube user 65scribe restores them using a solution of peroxide, xanthan gum, glycerine, an oxygen-based stain remover, and the power of sunlight. I'm not sure all those chemicals are easily available in the States (there's some discussion about that in the comments of this three year old video), but you'll be amazed at the results.
Check It Out: Removing the Yellow from a Vintage Mac Computer Case