When trying to change the background color of QToolButton widgets using this code:

QToolButton* button = new QToolButton(this);
QPalette p(button->palette());
p.setColor(QPalette::Button, QColor(Qt::black));
button->setPalette(p);

the background color of the button doesn't change when launching the application with "-style gtk". The Gtk theme used does not seem to matter at all (Clearlooks, Murrine, QtCurve-gtk, etc.)

It works fine with all other styles that come with Qt (Plastique, CDE, etc.) as well as KDE's Oxygen.