#include <QtGui>

class Widget : public QWidget
{
public:
    Widget(QWidget *parent = 0) : QWidget(parent)
    {
        resize(100, 100);
    }

    void paintEvent(QPaintEvent *)
    {
        QPainter p(this);

        p.fillRect(rect(), m_color);
    }

    void mouseMoveEvent(QMouseEvent *evt)
    {
        m_color = ColorUnderPos(QCursor::pos());

        update();
    }

    QColor ColorUnderPos(const QPoint &globalPt)
    {
        QColor color;

        const int screenNo = qApp->desktop()->screenNumber(globalPt);
        QWidget *desktopWidget = qApp->desktop()->screen(screenNo);

        const QImage screen = QPixmap::grabWindow(
            desktopWidget->winId(),
            globalPt.x(),
            globalPt.y(),
            1,
            1).toImage();

        const QRgb c = screen.pixel(QPoint(0, 0));
        color.setRgb(qRed(c), qGreen(c), qBlue(c));

        return color;
    }

    QColor m_color;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.show();

    return a.exec();
}