The calendar and task color in the synced google calendar are controlled by the online google account. Log in to your google account on a computer. Go into the Calendar (online) and change the colors there. You can pick any colors on that Gmail calendar.
I'm not sure how to change colors on non google calendars.(Well.. download the 'set calendar color') app from google play store--it seems to work for most calendars. But for gmail calendar-do it online.