Bleach works... but not normal household (clothing) bleach.
Go to your home improvement store and buy "DECK WASH". This stuff is more highly concentrated bleach and will definitely work wonders on faded, yellowed trackballs.
I have restored numerous balls. Most of them come out still glossy and very very close to the original ivory, not that horrible yellow blotchy color.
I soak it for about a week in a pickle jar, and swirling it around every 12 hours or so to keep it from getting a "dot" where it sits against the bottom/side too long.
It wont work on ones that are really really bad, but you'd be amazed. A second week, with new bleach will get it "slightly" better.
Be careful around the bleach. It's tough stuff. One drop will change blue to pink on any cotton you wear.