Grayscale image Color Conversion

[2lights]

The Inversion Color of image
Loading Image function

QFileDialog dialog2(this, "Upload secondary");
    dialog2.setNameFilter(tr("Images (*.png *.xpm *.jpg *.bmp)"));
    dialog2.setViewMode(QFileDialog::Detail);
 
    if(dialog2.exec())
    {
        imageFileName2 = dialog2.selectedFiles().first();
    }
 
    primaryImage.load(imageFileName1);
    primaryImage.invertPixels();

QFileDialog dialog2(this, "Upload secondary");
    dialog2.setNameFilter(tr("Images (*.png *.xpm *.jpg *.bmp)"));
    dialog2.setViewMode(QFileDialog::Detail);

    if(dialog2.exec())
    {
        imageFileName2 = dialog2.selectedFiles().first();
    }

    primaryImage.load(imageFileName1);
    primaryImage.invertPixels();

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In paint Function:

painter.drawImage(QPoint(0, 0), secondaryImage);

painter.drawImage(QPoint(0, 0), secondaryImage);

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The Changing color version of program ie(commented out until solution found)

for (int x = 0; x < secondaryImage.width(); ++x)
    {
        for (int y = 0; y < secondaryImage.height(); ++y)
        {
             if ( secondaryImage.pixel(x, y) == Qt::black)
                 secondaryImage.setPixel(x, y, QColor( 0, 0, 0, 0 )); //This line gives compiler error :confused:
             else
                  secondaryImage.setPixel( x, y, Qt::red ); //Nothing happens here :(
    }
    }

for (int x = 0; x < secondaryImage.width(); ++x)
    {
        for (int y = 0; y < secondaryImage.height(); ++y)
        {
             if ( secondaryImage.pixel(x, y) == Qt::black)
                 secondaryImage.setPixel(x, y, QColor( 0, 0, 0, 0 )); //This line gives compiler error :confused:
             else
                  secondaryImage.setPixel( x, y, Qt::red ); //Nothing happens here :(
    }
    }

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Ideas most welcome