To change the color of your event:
Go to your Weebly Editor page.
Click on the calendar and go to the Manage Events.
Click on an Event to edit it.
Scroll a bit on the same page and change the color in the Color section.
Please note if the event is synced, that the events synced from an external calendar, such as Google and Outlook, can only be edited on the original calendar. So, to change the color of synced events, go to the settings of the original calendar.