The colors associated with your calendar Events are dictated by their Space. Not to be confused with the Bookings in the Calendar. Please click here for more information on Calendar Colors.
To update a Space color:
Click on Settings on the left-hand sidebar
Select Organization
Click on Spaces
Find the space you wish to change the color for in the list
Click the Edit/Pencil icon on the right-hand side
Click on the Calendar Color
Select a new color
Click Save