Refurbing centre caps

Can anybody suggest the best way to refurb my pirelli P-slot centre caps?

One of them has turned a yellow colour and they all look generally tired.

Is there a particular cleaning agent that will make them sparkle again?

HI THERE U COULD TRY AUTOSOL METAL POLISH IT JUST DEPENDS HOW TARNISHED THEY ARE OTHER WAY IS BUY A BUFFING KIT THESE R ON EBAY 4 AROUND ?20 WILL FIT IN A NORMAL DRILL AND AWAY U GO

SEE POST ON METAL POLISHING IN THIS FORUM 2 GIVE U AN IDEA