Adam Pawlus from Galactichunter and 16bit.com answered a question about yellowing in his latest Q&A session:
http://www.galactichunter.com/gh/story/qa-star-wars-yellowing-and-pits"if only parts of the [2002-2003 releases like Clone Troopers, R2-D2, and some later ones like Commander Cody and the Vintage Saga Collection Scout Trooper action] figures turn yellow while other parts stay white, why aren’t they made entirely of the materials that don’t go yellow?
--Derek
This is the mystery of the ages! A lot of vintage 1980s figures have been seen with yellow torsos and white limbs, as well as white limbs and yellow torsos. Perhaps there's surgery to be done there.
Some of the figures from the 1970s through the 1990s turn yellow and while there are solutions like RetroBrite, they don't always work well or forever. The same kind of discoloration continues to happen, but around 2002 there's an added wrinkle - some of the discoloration isn't discolored plastic, but rather an oily residue that can be cleaned off using the right materials. I've had some "yellow" Saga troopers get cleaned up by wiping the limbs down with either a Mr. Clean Magic Eraser or used dryer sheets.
As to why Hasbro doesn't make figures that last forever, I don't think they know which batches of plastic will go sour until years later. It's possible the factory is using a lesser grade without Hasbro being able to know for years later. It's possible there's just no desire for them to consider long-term whiteness and brightness of their figures. Longevity is a topic that comes up a lot when I'm talking with Hasbro people on this and other brands - some materials just don't last well for the ages. Clear plastic can be brittle. Several kinds of chrome deco cracks and flakes over time - look at some Beast Machines releases. It isn't always pretty to see what can happen through the years. White plastic and clear plastic do tend to discolor with age, and while there are some gems that seem like they'll never change color, I don't tend to be the kind of person that wants to test that against the fury of the sun.
Even with the best of testing and intentions, some things just plain don't last. Mattel had a number of issues with the foamy material used in its Matty Collector Stay-Puft Marshmallow Man, which started to decay and discolor within a year - despite extensive testing to make sure the material was good. Was it a bad batch? Did the factory screw up? We may never know for sure. Collectors don't tend to talk a lot about older toys - and some stuff is packed away so we'll never see or realize damage was done for years."
