QPainter doesn't change color

Question

I'm learning Qt. I'm failing to realize the exercise of chapter 11 of Qt tutorial, which states "Change the color of the cannon when a shot is in the air." I chose to implement the change in paintCannon function (below). What's wrong with my code below?

void CannonField::paintCannon(QPainter &painter)
{
    painter.setPen(Qt::NoPen);
    if (autoShootTimer->isActive()){

        std::cout << "in paintCannon yellow; "  << std::endl; 
        // This gets called everytime `paintEvent` occurs. 
        // Please see the code in the web page (http://doc.trolltech.com/4.3/tutorial-t11-cannonfield-cpp.html) for this part.

        painter.setBrush(Qt::yellow);
    }else{
        std::cout << "in paintCannon blue; "  << std::endl;
        painter.setBrush(Qt::blue);
    }

    painter.save();
    painter.translate(0, height());
    painter.drawPie(QRect(-35, -35, 70, 70), 0, 90 * 16);
    painter.rotate(-currentAngle);
    painter.drawRect(barrelRect);
    painter.restore();
}

Since I first suspected Qpainter's save and restore might have been doing something wrong, I commented them out which ended up re-painting nothing.

Thanks.