What can I do to clean a minifigure's torso? I have the old General Grievous figure from 2005, and it is almost 10 years old. 

Due to its age, the torso now looks yellow-painted.

What can I do to clean it?
In general, legos can be un-yellowed by soaking in a combination of hydrogen peroxide, oxyclean powder (or sodium percarbonate), under sunlight or UV light. Or use Retr0Bright. Certain colors of painted decorations (such as gold) will not hold up well to the cleaning, but plain pieces and some colors do hold up well.