Here, the most colourful QColor starts at 255 instead of 0. Therefore, the code is

// constructor
FensterHaupt::FensterHaupt (int sz, QApplication *vorfahr) : scheibenzahl (sz) {
    uiFensterHaupt.setupUi (this);
    for (int h = 0; h <= 359; h += 60) {
    farbpalette.append (QColor::fromHsv (h, 255, 255) );
    }
    farbpalette.append (QColor (QColor::fromHsv (30, 255, 255) ) );
    spielfeldFullen ();
}

.