Just found your posting because I have the same problem with changing colours in KOrganiser. The colours also get changes on the calendar provider I have set up for my appointments not only in KOrganiser. I found out that the “digital Clock widget” (I hope it´s the correct name in English, in German it´s “digitale Uhr”) seems to be the cause of the problem.
When activating the events inside of the widget all the calendar colours change. They change randomly each time I use the drop down calendar.
Found no solution to the problem but to deactivate the events in the widget.
Greetings from Austria, broncheolus.