Until now i found a minimal working solution (-:
i am printing periodically the 20 default colors from Qt.

greetz Astronomy

MyPrintFunction()
{
// this are the numbers of the QT default colors
int QtColours[]= { 3,2,7,13,8,14,9, 15, 10, 16, 11, 17, 12, 18, 5, 4, 6, 19, 0, 1 };
int start=0;
int Index = 0;
QColor Color;

for(...search trough all data files...)
{
// read out Datafiles...

// Do calculation....

// and finally draw the Curves...
QwtPlotCurve *mycurces = new QwtPlotCurve(Filename);
#if QT_VERSION >= 0x040000
mycurces->setRenderHint(QwtPlotItem::RenderAntialiased);
#endif

// Set different colors...
Color.setRgb(QtColours[start]);
start++;
if(start == sizeof(QtColours))
{
startColour = 0;
}

mycurces->setPen(QPen(Color) );
mycurces->attach(manyGraphs);
mycurces->setData(Wavelength, Flux);

}