QGradient ColorInterpolation

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Hello,

i have some fundamental understanding question:
Assume i create a Lineargradient this way (not even sure this is correct)

QLinearGradient myIRGrad;
    myIRGrad.setStart(0, 0);
    myIRGrad.setFinalStop(0, 1);
    myIRGrad.setSpread(QLinearGradient::PadSpread);
    myIRGrad.setColorAt(0, Qt::red);
    myIRGrad.setColorAt(0.5, Qt::yellow);
    myIRGrad.setColorAt(0.9, Qt::white); 

How i can get the interpolated color back at point, let say 0.333 ?
The great qt docs are rather poor here or i search not smart enough.
I expcted something like this:

qDebug() <<  myIRGrad.ColorAt(0.333);

but this does not work of course.
I think i 'm on a total wrong way and need soemthing like QColormap or similar

Any help highly welcome, thank you