java frame not changing colour

I am unable to change the colour of my frame in Java code below. I know the code to change it would be frame.getContentPane().setBackground(Color.gray); However, this is not working for me, I am not sure what is the reason behind this, but if you are able to solve the problem do let me know, thank you.

public class UserLoginPage implements ActionListener {
//Put all JLabels,Frames and buttons here etc
JPanel panel = new JPanel();
JFrame frame = new JFrame();
JLabel userLabel = new JLabel("Username");
JLabel passwordLabel = new JLabel("Password");
JTextField userText = new JTextField();
JTextField passwordText = new JTextField();
JButton loginButton = new JButton("Login");
//Label for successful login
JLabel success = new JLabel();
//Default Constructor to add the frames and panels etc
public UserLoginPage(){
panel.setLayout(null);
userLabel.setBounds(10,20,80,25);
panel.add(userLabel);
passwordLabel.setBounds(10,50,80,25);
panel.add(passwordLabel);
userText.setBounds(100,20,165,25);
panel.add(userText);
passwordText.setBounds(100,50,165,25);
panel.add(passwordText);
loginButton.setBounds(10,80,80,25);
loginButton.addActionListener(this);
panel.add(loginButton);
success.setBounds(10,110,300,25);
panel.add(success);
//success.setText();
frame.setSize(500,500);
frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
frame.getContentPane().setBackground(Color.gray);
frame.setVisible(true);
frame.add(panel);
}
public static void main(String[] args){
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
new UserLoginPage();
}
});
}
@Override
public void actionPerformed(ActionEvent e) {
String user = userText.getText();
String password = passwordText.getText();
System.out.println(user + ", " + password);
if(user.equals("Jackson") && password.equals("1234")){
JOptionPane.showMessageDialog(frame,"Login successful");
}
else{
JOptionPane.showMessageDialog(frame,"Invalid password or username");
}
}
}

You're adding panel, a JPanel to your JFrame, and since JFrame's contentPane uses a BorderLayout, this JPanel (which is opaque), will completely cover the contentPane, preventing visualization of the contentPane's background.

Solution:

• Either make the panel not-opaque via panel.setOpaque(false); so that now its container's colors or images will show through
• or leave it default opaque give it and not the contentPane the background color of choice.

Unrelated issue is here:

panel.setLayout(null)

You really don't want to be doing this for many reasons, including because this will make your GUI work well / look nice on only one platform. It also makes it very hard to upgrade and enhance the GUI later.

For example, and incorporating some of Andrew Thompson's suggestions, here is an example login GUI that is geared towards creating a JPanel, one that in this example is placed into a modal JDialog (similar to a JOptionPane but with more flexibility) and displayed. The code uses GridBagLayout along with GridBagConstraints when adding components to place them where I want them to be in a pleasing way that works on all platforms, and that allows ease and flexibility should I want to add more components:

import java.awt.Color;
import java.awt.Component;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.Window;
import java.awt.Dialog.ModalityType;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import javax.swing.*;
@SuppressWarnings("serial")
public class UserLoginPanel extends JPanel {
private static final Color BACKGROUND = new Color(200, 200, 200);
private JTextField userText = new JTextField(15);
private JPasswordField passwordText = new JPasswordField(15);
LoginAction loginAction = new LoginAction(this, "Login", KeyEvent.VK_L);
JButton loginButton = new JButton(loginAction);
public UserLoginPanel() {
super(new GridBagLayout());
setBackground(BACKGROUND);
GridBagConstraints gbc = new GridBagConstraints();
gbc.gridx = 0; 
gbc.gridy = 0;
gbc.fill = GridBagConstraints.HORIZONTAL;
gbc.weightx = 1.0;
gbc.weighty = 1.0;
int insetGap = 4;
gbc.insets = new Insets(insetGap, insetGap, insetGap, insetGap);
add(new JLabel("User Name:"), gbc);
gbc.gridx = 1;
add(userText, gbc);
gbc.gridx = 0;
gbc.gridy = 1;
add(new JLabel("Password"), gbc);
gbc.gridx = 1;
add(passwordText, gbc);
gbc.gridx = 0;
gbc.gridy = 2;
gbc.gridwidth = 2;
add(loginButton, gbc);
insetGap = 8;
setBorder(BorderFactory.createEmptyBorder(insetGap, insetGap, insetGap, insetGap));
}
public String getUserName() {
return userText.getText();
}
public char[] getPassword() {
return passwordText.getPassword();
}
public static void main(String[] args) {
UserLoginPanel loginPanel = new UserLoginPanel();
JDialog dialog = new JDialog(null, "User Login", ModalityType.APPLICATION_MODAL);
dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
dialog.add(loginPanel);
dialog.pack();
dialog.setLocationByPlatform(true);
dialog.setVisible(true);
}
}

<!-- -->

@SuppressWarnings("serial")
class LoginAction extends AbstractAction {
private UserLoginPanel loginPanel;
public LoginAction(UserLoginPanel loginPanel, String name, int mnemonic) {
super(name);
putValue(MNEMONIC_KEY, KeyEvent.VK_L);
this.loginPanel = loginPanel;
}
@Override
public void actionPerformed(ActionEvent e) {
String userName = loginPanel.getUserName();
char[] password = loginPanel.getPassword();
System.out.println("User Name: " + userName);
System.out.println("Password:  " + new String(password));
Component source = (Component) e.getSource();
if (source != null) {
Window window = SwingUtilities.getWindowAncestor(source);
if (window != null) {
window.dispose();
}
}
}
}