QBrush
is what controls the fill color of your ellipse. In the code you've provided, you're just giving a brush with a solid pattern (hence the black fill).
If you look at the various QBrush constructors, you'll note that there are several different kinds. The ones you'll likely be most interested in are
QBrush ( Qt::GlobalColor color, Qt::BrushStyle style = Qt::SolidPattern )
QBrush ( const QColor & color, Qt::BrushStyle style = Qt::SolidPattern )
which will allow you do do things like:
scene->addEllipse( x, y, size, size, pen, QBrush(Qt::red) );
or
scene->addEllipse( x, y, size, size, pen, QBrush(QColor("#FFCCDD") );
See Qt's QBrush documentation