I have answered this question in another forum post (https://forum.mendix.com/link/questions/111806)
I think it would work for your use case too. In short you should add a “Color” Attribute to the Event Object. Then use an event listener to set the color if a certain condition is true. In my answer it was with enum values but you can just compare to a boolean.
I don’t have the actual change object activity anymore as I have just created it for that example a while ago. I just took one change object activity in my open active project.
This is what it should be like:
Select your event object (in this picture “kurs”) in the “Input” section and then change the “color” attribute (in this picture “Kursname”) to the hex color value that you want to give to the event object. Don’t forget to select “Commit: Yes” and “Refresh in Client: Yes”.