If you add personal calendars to iCal, the calendars' colors are used in iCal. If you change the calendars' colors in iCal, the calendar colors in the web calendar change to match.
To apply a different calendar color:
While viewing a personal or group calendar, hold the mouse cursor over the calendar that you want to apply a different color to and then click the Information (i) button.
Choose a different color from the Calendar Color pop-up menu.