Events in Google Calendar are color coded. Each calendar has also a default color that is assigned to all your events unless you set a specific one when creating a new event.
Google Calendar Pro add-on for Simple Calendar has an option to display events colors that you have set in Google.
When you tick this option, depending on the calendar type you are using, the event colors from Google will be displayed along the events displayed with Simple Calendar. If no specific color is used in Google for an event, the default calendar color will be used.
In order to find the new event color setting you will need to make sure you are using the new Google Calendar Pro event source option.
There is one important concept to understand with Google though: calendar and event colors are set individually for each user subscribing a calendar or an event. When you share, for example, a calendar with different people, or when others sign up for an event, they might set a different color on their own version of the event as it appears on their own Google Calendar app. Google Calendar Pro add-on only synchronizes with the Google Calendar belonging to the Google user that performed the authentication when setting up the plugin in WordPress, therefore only the corresponding colors for this particular user will be used.