---

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

public class Main extends JPanel {

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        for (int i = 0; i < 4; i++) {
            for (int j = 0; j < 4; j++) {
                int c = (i + j) * 255 / 6;
                g.setColor(new Color(c, c, c));
                g.fillRect(i * getWidth() / 4,
                           j * getHeight() / 4,
                           getWidth() / 4,
                           getHeight() / 4);
            }
        }
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Градиент");
        Main panel = new Main();

        frame.add(panel);
        frame.setSize(400, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}