mine's silver and am allergic to the metal so 'tis covered in clear plastic. I would prefer clear plastic, green where you can see the innards working.
Black is easier on the eye though with less chance of eye-strain
actually the white one is faster because white reflects the heat so they can hike up the clockspeed a few notches
however on the downside - people may mistake it for a shrunken mac or a big iPod - so you'll loose some street cred - because people will assume you've bought it as a fashion statement rather than as a computer.