You can create other calendars in addition to your main calendar. For example, you could create a separate calendar for things such as doctor’s appointments that you do not want on your main calendar. All of the calendars you create will be listed under "My Calendars" on the left side of your page.
To create a new calendar:

You can change the color associated with any of your calendars. Click the down arrow next to the appropriate calendar's name in the “My Calendars” list on the left, and select a new color from the menu that appears.

Please note that there's a limit to the number of calendars you can create at one time. If you're receiving an error message when creating additional calendars, please wait 24 hours before trying again.
For more information, visit
http://www.google.com/support/calendar/bin/answer.py?hl=en&answer=37095