Well, the way Outlook/exchange seems to do it is to just use the color of the
last category you added to the appointment in the calendar view. When you
open the details of the appointment, however, all the categories, with their
associated colors, are shown at the top of the event.
Interestingly, when you use multiple categories on an e-mail message, a
little color icon appears next to the message for each category you add, so
if you have only 1 category, there will only be one color next to it, but if
you add 3, there will be 3 smaller categories next to it. I'd argue that the
same type of thing should be used for appointments, so that the appointment
is split into multiple colors when it is assigned multiple categories. As it
is, however, isn't a horrible solution as you can still use multiple
categories but specify by adding last the most relevant category to show up
in the calendar views. Personally, I avoid using multiple categories anyway
as I sync to a Palm device which only supports 1 category/appointment. This
is why a multiple calendar approach would also work for me if I was happier
with the way Outlook displays them. However, I can see where multiple
categories could be a useful way to organize things, especially since you can
use search or different views to highlight particular categories. In this
case an event will show up in different views/searches without having to
actually make duplicates of the event.
