Have a color drop down or selection box in each of the calendar forms so that when the user create a calendar entry, they can also select what color they want the entry to have in the calendar views in addition to what is available today in the Preferences settings.
For example:
I’ve created an appointment calendar entry and I want this one appointment calendar entry to stand out so I want to have the option of selecting for just this one calendar entry a different color from what I have set in my Preferences for appointment calendar entries…