The next simple application:

public class Undecorated {
    public static void main(String[] args) throws Exception {
        SwingUtilities.invokeAndWait(() -> {
            JFrame frame = new JFrame();
            frame.setUndecorated(true);
            frame.setSize(100, 100);
            Container content = frame.getContentPane();
            content.add(new JButton("test"), BorderLayout.NORTH);
            content.setBackground(Color.red);
            frame.setVisible(true);
        });
    }
}

produces a frame with gray background on OEL7. No Swing controls are painted.