Calendar colors work for me, they just don't really work in the preview, haven't played around with categories yet, but in the preview/editing screens, the colors are broken here too.

I am maybe the only person who completely manages his calendar on the N900 itself, so I never even bothered with Outlook (which I don't have anyways), Thunderbird/Sunbird or some Google stuff. So I can't really tell anything about this matter.