Wow, I've been looking for a forum like this for quite awhile, I have been collecting again for six months, although I had a good start as I had all of my childhood collection intact, Mainly started to upgrade and became addicted.  anyway.  I have discovered that letting your figures and soiled parts soak in a mega dose oxyclean and water works miracles, I dare say it will make a yellow figure a shade whiter, but its probably self deception on my part.   It will also remove ink, stage blood, and whatever else little whipper snappers tortured their figures with if you have the patience and want to scrub judiciously. I removed red stains from a land of the jawas playset base that soap and water would not touch.  Just this evening I soaked a survival kit backpack that was almost black with worn in grime, one that soap and water did nothing to, and it looks almost as good as new!  I forgot, it does wonders on soiled/stained cloaks as well, I have a royal guard that the colors were brought out amazingly.  GREAT STUFF.
WARNING: dont soak a rancor keeper, his belly will fill with water, also make sure to rinse with normal water a few times , or oxyclean residue will show up as dry flakes, they do brush off however.