It would really make categories more useful if you could associate an icon with them which would be shown in a calendar event/task of that category. In general, Mozilla Calendar should make more use of icons. Colors are not enough, especially not when you have 50+ categories.
Created attachment 242817 [details] Basic example of what I mean The category icon should be displayed within the calendar entry. Ideally, it would be a scalable vector graphics that would be guaranteed to fit. Also, it could just be a watermark-like icon that just adjusted the background color using e.g. gamma and shadows. (Gimp can really be a pain in the ass sometimes. Hopefully the end result would look much more polished than this.)