"Color Coded Events lets you assign specific colors to certain events: put pink on your daughter's soccer practice or make your lunch dates red. It's a great way to stay organized, keep track of recurring events, and add a little flair to your calendar. Color coding is private to you and anybody who can edit your calendar," explains Google.
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If you want even more options, go to Google Calendar Labs and enable "Event flair" to be able to add icons to your events.
{ Thanks, Brahm. }