You can color-coordinate events in iCal, you might have to do it from the computer first though. Then sync it to the iPod.

EDIT: Or, if she doesn't like iCal, you can always try out Things. It's a FANTASTIC application that also has an iPod application to sync with the desktop version. Unfortunately, the iPod app isn't free, I believe it's around $10 and the desktop app is about $50. However, I think they are worth the price.