[robg adds: Mac Help has some information on this, though on my machine, it's not entirely true:
Mailboxes that contain both messages and other mailboxes are blue. Mailboxes that contain only other mailboxes are colorless.On my machine, I have a top-level blue folder that contains only sub-folders. I also have a top-level white folder that only contains sub-folders, so I'm not sure why they're appearing differently. This hint, however, does explain how to create a white (colorless) folder, if you so desire...]

