Welcome to Android Central! Are you trying to change the color of individual events, or the color of all events from a particular calendar in your Google account?
If you're trying to set a color for a specific event, are you sure you're creating the event in one of your Google calendars, and not the local Phone account's calendar? You can set the color of the event in a Google calendar, but not in the local Phone account calendar. When you create a new event, look at the associated calendar at the top, just under "Add title." If it says "My Calendar," that's probably your Phone account calendar. Tap it, and choose one of the calendars associated with your Google account instead.