Calendar Event Color
The colors are set in your Google calender (the Google web site).

There you can set the individual colors for the events.



The Mac Calender is using the Google calender settings, but you can not change the colors in your Mac Calender.

UPDATE

If you really want to use the Apple Calender there is a somewhat initially complicated process to set it up, but easy to use afterward.

You would in Google Calender make new Calenders (name them to your liking) like Events A ect.. then assign a color to it. 

Now they will show in your Apple calender with the assigned color.



and you will see them in your calendar list