I often use the ArmourAll on my laptops. I've also thought that you might be able to use regulation Rubbing Compound, like you use on a cars paint-job, but I haven't tried it because I no longer own a power-buffer, and I ain't about to try doing it by hand. I always used a product called 'Black Magic'. It's a liquid-type compound, not a paste, and it works great on laquer or enamel, and would probably make short work of discolored plastic. OTOH, you might need to go with a coarser grit, if the discoloration goes very deep into the plastic, then finish it with finer stuff.
--T