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import javax.swing.*;

class First{
    public static void main(String args[]){

        Second gui = new Second();
        gui.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        gui.setSize(425, 150);
        gui.setVisible(true);
    }
}

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.util.Formatter;
import java.util.Scanner;

public class Second extends JFrame {

    private JButton b;
    private Color color = Color.WHITE;
    private JPanel panel;

    public Second(){
        super("Title");
        panel = new JPanel();
        panel.setBackground(color);

        b = new JButton("Choose a color");
        b.addActionListener(
                new ActionListener() {
                    @Override
                    public void actionPerformed(ActionEvent e) {
                        color = JColorChooser.showDialog(null, "Choose a color", color);
                            panel.setBackground(color);
                        }
                }
        );

        add(panel, BorderLayout.CENTER);
        add(b, BorderLayout.SOUTH);
    }

}