If you review the documentation for that widget, you will find information about how to set the color of an event based on enumerations in the entity that contains the events. Maybe that's a good starting point for you.
Currently, there are two calendar widgets available. I'm using the somewhat older one. Solution when using this widget:
First you need a color enumeration in your Event entity. You can change this in your domain model. You also need to be able to select the color on your ‘Event_NewEdit’ page.
Once the calendar widget has been dragged in place, I open the settings by double clicking.
Go to the tab ‘View settings’. At the bottom it's possible to select your color enumeration. Under the enumeration, you can add new colors. (Check the image below).
I hope this helps :).