Unfortunately, the figure on the left is indeed suffering from discolouring / 'yellowing', which is actually a chemical change in the plastic. This sadly happens to figures sometimes (and in both the vintage and modern Star Wars lines), with the torso and or one or more limbs changing colour from the original shade Kenner intended. Some reports I've read blame bromination of the plastic, where the presence of bromine in the plastic creates a distinctive 'yellowing' or sometimes 'browning' of the colour. As far as I know, there is no way to permanently reverse it. Hydrogen peroxide and sunlight can sometimes temporarily lessen or even remove 'yellowing' in some cases, but be warned that this will reverse over time, returning to its 'yellowed' state and may well also damage the plastic to boot.
Sadly, these figures were created as toys, rather than collectables with a multi-decade shelf-life, and as such, Kenner gave little thought to the longevity of the plastic or its colour degradation over time.
Hope that helps.
Jeremy
