JLabel.setBackground（Color color） 不起作用？

在此 SSCCE 代码中：

此方法有效

label.setForeground(Color.GREEN);

但是下一个方法不起作用！

label.setBackground(Color.BLUE);

import java.awt.*;
import javax.swing.*;

public class LabelColorTest extends JPanel
{

    static JLabel label;
    JPanel panel;

    public LabelColorTest()
    {
        label = new JLabel();
        label.setVerticalAlignment(JLabel.CENTER);
        label.setHorizontalAlignment(JLabel.CENTER);
        label.setText("Hello world");

        panel = new JPanel();
        panel.setLayout(new BorderLayout());
        panel.add(label, BorderLayout.CENTER);

        label.setForeground(Color.GREEN);  //HERE
        label.setBackground(Color.BLUE);  //HERE


        this.setLayout(new BorderLayout());
        this.add(panel);
    }

    private static void createAndShowGUI()
    {
        JFrame frame = new JFrame("Hellow world");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 250);
        frame.add(new LabelColorTest(), BorderLayout.CENTER);
        frame.setVisible(true);
    }

    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                UIManager.put("swing.boldMetal", Boolean.FALSE);
                createAndShowGUI();

            }
        });
    }
}