Google Calendar has included the ability to add colours to particular events and change your overall colour scheme since May last year, but you were stuck with Google's selection. If you have a particular colour vision in mind, you can now customise the colours yourself.
To change the background colour, hover over the name of the calendar in the left sidebar, click on the downward-pointing arrow, then click 'choose custom colour' in the menu that appears. You can change the background colour to anything you like, but text can only be 'light' or 'dark'. Google's blog post announcing the change also suggests that you can customise individual event colours, but I couldn't access that option on my calendar.
[via Google Operating System]