Panel

package com.company;

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

public class Panel extends JPanel {
    private  int value = 0;

    public  Panel() {
        setLayout(null);

        Timer timer = new Timer(200, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                value++;
                if (value == 256)
                    value = 0;
                repaint();
            }
        });
        timer.start();
    }

     public void paintComponent(Graphics graphics) {
        super.paintComponent(graphics);

        for (int i = 0; i <= 255; i++) {
            graphics.setColor(new Color(0, (i * value) % 255, 0));
            graphics.drawRect(250 - i / 2, 220 - i / 2, i, i);
        }
     }
}