Source: http://lifehacker.com/5799957/google-calendar-adds-color-coding-to-individual-events
If you use Google Calendar, you probably use multiple calendars to color-code your events for easy organization. Now, Google's rolling out a new feature that lets you color code individual events, instead of just entire calendars. That way, you can keep events in a specific category, but draw more attention to them with a specific color. Just click the box to the left of the title in an event and choose the color for that event. If it's a recurring event, you can change all occurrences or just that one occurrence. The feature seems to be rolling out gradually, so if you don't have it yet, check back periodically and see if it shows up. [Google Calendar Help via Google Operating System]