This URL would modify the calendar preference ceBgcolor to have the value white:
http://calendarserver/set_useprefs.wcap?id=b5q2o8ve2rk02nv9t6 &set_attrs=ceBgcolor=white
This URL would allow the logged-in administrator to modify the calendar preference ceBgcolor to have the value black for user jdoe:
http://calendarserver/set_userprefs.wcap ?id=b5q2o8ve2rk02nv9t6 &userid=jdoe &set_attrs=ceBgcolor=black