Last year Google added a new color palette for Google Calendar and many users complained. Some of them thought that the new calendar colors make it difficult to tell events apart, while other people wrote that they're too muted.
Now you can customize calendar colors. Just click the arrow icon next to a calendar in the left sidebar, click "choose custom color" and pick your favorite background color. Select "light text" if the text is hard to read.
"Google Calendar users have had the ability to change the colors of specific events or calendars from a default color palette. Users can now choose a custom color if the default palette does not meet their needs," informs Google. This feature is also available for Google Apps.