Multiple JPanels in one JFrame not showing top panel

So I'm writing a program in which I wish to have a single JFrame containing a JPanel header in a separate colour and directly underneath have a grid of buttons in a separate JPanel. So far my program works perfectly except for the fact that the header String isn't showing up in the NORTH panel. Instead I'm getting a box containing the set background colour with a small grey box in the centre. I'm wondering if I didn't set the size of the panel correctly?

I have heard this can be accomplished using JLabel, but when I tried to do this, it would not show the background colour that I had set.

So, could anyone please show me how to achieve the following either with the JPanel (preferably because I would like to know how it works and what I'm missing) or with JLabel: filling that little grey box in the middle of the header with a String.

Here is my code:

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

public class Example {

    public static void main(String[] args) {

        // Initialize a panel for the header, and mainGrid which will contain buttons
        JPanel panel = new JPanel();
        JPanel header = new JPanel();
        JPanel mainGrid = new JPanel();

        // Initialize the header
        DisplayPanel message = new DisplayPanel();
        header.setBackground(Color.ORANGE);
        header.add(message);

        // Initialize the mainGrid panel
        mainGrid.setLayout(new GridLayout(2,2,2,2));
        mainGrid.add(new JButton("1"));
        mainGrid.add(new JButton("2"));
        mainGrid.add(new JButton("3"));
        mainGrid.add(new JButton("4"));

        // Add the two subpanels to the main panel
        panel.setLayout(new BorderLayout());
        panel.add(header, BorderLayout.NORTH); // The issue is this panel isn't displaying the String created in DisplayPanel
        panel.add(mainGrid, BorderLayout.CENTER);

        // Add main panel to JFrame
        JFrame display = new JFrame("Test");
        display.setContentPane(panel);
        display.setSize(200,100);
        display.setLocation(500,200);
        display.setVisible(true);
        display.setResizable(false);
        display.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    private static class DisplayPanel extends JPanel {
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawString("header" , 20, 20); // The string I want to be displayed
        }
    }
}

I would very much appreciate anyone's help or input as I have only been studying Java for a few months and this is my first post. Thank you in advance.

Also, any general tips on writing that you may have would be greatly appreciated.