Thanks for changing the title for me.
As for the Qt-CalDAV thing - I totally understand that CalDAV doesn’t have anything to do with Qt or the way it interprets color values. Qt also doesn’t have any idea about CalDAV. If you asked me, then I would say that this is just a total coincidence that Korganizer correctly displays calendar colors sent by the Runbox calendar service. The support for SVG color names was implemented in QColor for totally different reasons, not with Apple’s calendar-color in mind (Qt as a programming framework just supports SVG as one of many graphics formats, that’s why QColor needs to understand SVG color names). QColor, of course, supports also RGB(A) color values. And most probably Runbox chose to use SVG color names for their calendars because it was just more convenient for them (we give users 120 predefined SVG colors for their calendars - they be happy; we have less work - we be happy as well).
Unfortunately, I also have no knowledge of the Apple’s calendar-color specification. From the DAVx5 logs I just assumed that these color names are Apple-defined and are part of their calendar-color specification. Turned out that my assumption was wrong. It seems that this caused a lot of unnecessary confusion 😀