If I had to choose, I'd pick red. Ditto all the comments about going for an awesome cause.
One thing you may think about is this:
http://www.colorwarepc.com/products/NanoCombo_Html_colorize.aspx
Then you can make it any color you want! Their reputation is incredible. It was actually on Uncrate's top ten upgrades for the macbook found here:
http://www.uncrate.com/men/gear/laptops/pimp-your-macbook/
Holy expensive. i mean awesome that they do that, but 75 bucks to paint your nano? hardly worth it. 450 for a macbook painted?! aaaaah did i miss something or is paint getting expensive now adays..