Hi there, Andy!
Good question. This is a way to do this on the main calendar, however I don’t think the same sort of tactic would work for the min-calendar widget.
The problem is that the mini-calendar always displays one dot, even if there are multiple events in the day. So, for example, if there are multiple events are varying categories, it would be tough to determine which color to assign to that day.
Sorry this probably isn’t the answer you were hoping for, but does it at least help? Please let me know. 🙂
Cheers!
Geoff