Picture

[kshegunov]

I suggest you average the color in the rectangle, e.g. (thanks to @VRonin for the scaffolding):

QColor dominantColour(const QImage & image,const QPoint & topLeft, const QSize & rectSize)
{
    qint32 averageRed = 0, averageGreen = 0, averageBlue = 0;

    const qint32 maxX = qMin<qint32>(image.width(), topLeft.x() + rectSize.width());
    const qint32 maxY = qMin<qint32>(image.height(), topLeft.y() + rectSize.height());
    for (qint32 y = topLeft.y(); y < maxY; y++)  {
        for (qint32 x = topLeft.x(); x < maxX; x++)   {
            QRgb pixel = image.pixel(x, y);
            
            averageRed += qRed(pixel);
            averageGreen += qGreen(pixel);
            averageBlue += qBlue(pixel);
        }
    }

    qint32 n = rectSize.width() * rectSize.height();

    Q_ASSERT(n);
    if (n <= 0)
        return Qt::black;

    return QColor(averageRed / n, averageGreen / n, averageBlue / n);
}

[Payx]

Thank you very much !