Does camera body color affect performance, or is black mostly for appearance?

I think black is primarily chosen to be discrete, and because that's what people expect. Whilst some people like to be different, most want their camera to look like a camera. On the first point, a shiny camera would be a liability for nature photography, as light reflecting off it could scare away animals. People tend to use camo coverings over Canon's white lenses for this reason.

It's not a manufacturing limitation: there are plenty of silver plastic bodies, in the good ol' days bare metal was popular, and then there's Pentax ;)

The other argument comes down to temperature issues. Black absorbs the most solar energy, but also radiates the most heat energy back into the atmosphere so the problem isn't as bad as you might think.

Canon's supertelephoto lenses have traditionally been white (cream) and the reason given was to prevent heat expansion affecting the optics. Other manufacturers predominantly make back superteles (actually Nikon once made a 300 f/2.0 that was white), as Tzarium points out in the comments this may be due to their not using flourite elements and thus have fewer issues with heat expansion.

(source: jplnet.com)

A white Nikon lens!

In the end then it comes down to marketing. White is Canon's trademark — when you see a sea of white lenses at a sporting event you know the majority of shooters are using Canon and that's a fantastic advert for them!

(source: theoneaboutus.net)

http://viz.cwrl.utexas.edu/files/CanonShades.jpg

(source: radiantlite.com)