painting problem

[K4ELO]

I have a signal plot with a grid where I would like to change the background color of just some of the grid cells.
Tried this:

void SignalView::paintEvent(QPaintEvent*)
{
    QPainter* painter = new QPainter(this);
 
    QColor bpColor;
    bpColor.setNamedColor("grey");
 
    painter->setPen(bpColor);
    QwtPainter::drawRect(painter, 500, 500, 100, 500);
}

void SignalView::paintEvent(QPaintEvent*)
{
    QPainter* painter = new QPainter(this);

    QColor bpColor;
    bpColor.setNamedColor("grey");

    painter->setPen(bpColor);
    QwtPainter::drawRect(painter, 500, 500, 100, 500);
}

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But no paint and the message QPaintDevice: Cannot destroy paint device that is being painted appears in the application output window.

Any suggestions are welcome.