Google has update the color palette of Google Docs Calendar, now letting you choose from 24 colors that match the new interface. The new colors are only available in the new UI of the Google Calendar.
To switch to the new interface, click the Options menu in the navigation bar and select "Try the new look". You can always go back to the old UI from the same menu.
"The color of each of your calendars has been updated to use the closest possible color from the new palette. The new colors will only be available in the web interface (for example, you won't see these colors if you're accessing Calendar via your mobile phone)," informs Google.
Here are some pictures of the new color palate UI:
[Thanks, Cougar, Alex]