background color for QGraphicsWidget

[QAir]

Hello all,
I'm trying the following to set background color for a QGrahpicsWidget object

QGrahpicsWidget* w = new QGrahpicsWidget();
QBrush brush;
QPalette palette;
palette = w->palette();
brush.setColor(Qt::blue);
palette.setBrush(QPalette::Active, QPalette::Background, brush);
w->setPalette(palette);

QGrahpicsWidget* w = new QGrahpicsWidget();
QBrush brush;
QPalette palette;
palette = w->palette();
brush.setColor(Qt::blue);
palette.setBrush(QPalette::Active, QPalette::Background, brush);
w->setPalette(palette);

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but it doesn't work
any one has an idea???
thx to much