For white toys, such as stormtroopers etc.. the best results for me have been simply sitting the items in the sunlight and turning them over and over.
dealing with pigmented plastics is a whole new game. the oils in many plastic toys will evaporate from the surface, over time.. the only way I think that may work for some items, is to lift the deeper remaining pigment up to the surface with a blow-dryer or heat gun. an oven, or the sun. Please see some of the youtoob vids about restoring or reclaiming car plastics bumper and dashboard colors with a heat-gun before attempting to reclaim colors on toys!!
and always practice on a test subject, so you don't do damage to your good items.
Vintage carded figure bubble yellowing and staining are impurities in the plastic formulation and as I understand it, these can never be cleaned or reclaimed.
Good Luck !