No not working ,
rubberBand->setPalette(Qt::transparent); removes the color from borders also..

not getting setPen in QPalette

You should not use setBrush, it will fill your rectangle with yellow color, Use setPen with yellow color, it will set the yellow border.

Rectangle A QRubberBand can represent a rectangle. Some
styles will interpret this as a filled (often
semi-transparent) rectangle, or a rectangular
outline.

It's also filled on windows 10 so I guess it's by design / also affected by the platform style.

It draws with QStyle::CE_RubberBand
so you could override with http://doc.qt.io/qt-5/qproxystyle.html

class MRubberStyle : public QProxyStyle {
 public:
  MRubberStyle() : QProxyStyle() {}
  ~MRubberStyle() {}

  virtual void drawControl(ControlElement element, const QStyleOption* option, QPainter* painter, const QWidget* widget = 0) const {
    switch(element) {
      case CE_RubberBand:
        // draw as u want
        return;
      default:
        QApplication::style()->drawControl(element, option, painter, widget);
    }
  }
};
...
 app.setStyle( new MRubberStyle );

dummyband = new QRubberBand(QRubberBand::Rectangle, this);

QPalette palette;
//palette.setBrush(QPalette::Foreground, QBrush(color));
//palette.setBrush(QPalette::Base, QBrush(color));
palette.setBrush(QPalette::Highlight, QBrush(yellow));
dummyband->setPalette(palette);
dummyband->setGeometry(QRect(topLeft, bottomRight));
dummyband->show();

Could you show your code that sets the color to Yellow?

Eddy