Repaint fails when using threads

Hi, I'm trying to get to grips with threads and am creating a test application that carries out several tasks simultaneously. My problem is that when using a thread, the repaint() method doesn't seem to update the panel that I would expect it to. Below is the fragment of code in question:

class ColourPanel extends JPanel implements Runnable {

public void run() {
try {
for (int i=0;i<100000;i++) {

randomNumber = Math.random();
if ((randomNumber >= 0) && (randomNumber < 0.25))
setBackground(Color.RED);
if ((randomNumber >= 0.25) && (randomNumber < 0.5))
setBackground(Color.BLUE);
if ((randomNumber >= 0.5) && (randomNumber < 0.75))
setBackground(Color.MAGENTA);
if ((randomNumber >= 0.75) && (randomNumber <= 1))
setBackground(Color.ORANGE);

repaint();
setVisible(true);
Thread.sleep(200);
}
}
catch (InterruptedException e) {
}
}

public ColourPanel() {

setLayout(new BorderLayout());

colourButton = new JButton("Change Background");
add(colourButton, BorderLayout.SOUTH);

colourButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {

Runnable q = new ColourPanel();
Thread s = new Thread(q);
s.start();

}
});
}

private JButton colourButton;
private double randomNumber;

}

I would expect the repaint() method to update the panel with the new background colour. However, this does not work and I can't see why. I have tried calling getParent().repaint() and getRootPane().repaint() but these also don't work. Am I barking up the wrong tree? Any help would be greatly appreciated!

Code:

---

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

public class ColourPanel extends JPanel implements Runnable {

    public void run() {
        try {
            for (int i=0; i<10; i++) {
                randomNumber = Math.random();
                if ((randomNumber >= 0) && (randomNumber < 0.25))
                    setBackground(Color.RED);
                if ((randomNumber >= 0.25) && (randomNumber < 0.5))
                    setBackground(Color.BLUE);
                if ((randomNumber >= 0.5) && (randomNumber < 0.75))
                    setBackground(Color.MAGENTA);
                if ((randomNumber >= 0.75) && (randomNumber <= 1))
                    setBackground(Color.ORANGE);

                repaint();
                System.out.printf("i = %s  isDisplayable = %b%n",
                                  i, isDisplayable());
                Thread.sleep(200);
            }
        } catch (InterruptedException e) {
            return;
        }
    }

    public ColourPanel() {
        setLayout(new BorderLayout());
        colourButton = new JButton("Change Background");
        add(colourButton, BorderLayout.SOUTH);

        colourButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                // This first line creates a new instance of ColourPanel
                // which is not the current instance that has been
                // instantiated and loaded into a gui. So this new
                // instance runs just fine but cannot be seen.
                //Runnable q = new ColourPanel();

                // If you get a reference to the current instance
                // that has been instantiated and added to a gui
                // you will see the colors change. Since we are inside
                // another class ("this" == this ActionListener anonymous
                // class) we use the static form of obtaining a reference.
                Runnable q = ColourPanel.this;
                Thread s = new Thread(q);
                s.start();
            }
        });
    }

    public static void main(String[] args) {
        ColourPanel panel = new ColourPanel();
        panel.setPreferredSize(new Dimension(200,200));
        JOptionPane.showMessageDialog(null, panel, "", -1);
    }

    private JButton colourButton;
    private double randomNumber;
}

---