import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class ColoredBoxes2 {

public static void main(String[] args) {
new ColoredBoxes2();
}

public ColoredBoxes2() {
EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}

JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new TestPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}

public static class TestPane extends JPanel {

// dimensions of the box
// changed rows to 20 to make a square
protected static final int ROWS = 20;
protected static final int COLS = 20;
protected static final int BOX_SIZE = 30;

private List<Color> colors;
// initialize the 2D array
Color[][] myColor = new Color[ROWS][COLS];

public TestPane() {
    int length = ROWS * COLS;
    colors = new ArrayList<>(length);

    for (int row = 0; row < ROWS; row++) {
        for (int col = 0; col < COLS; col++) {

        // rgb triple values
        // initialize them randomly to get random colors
        int c1 = (int) (Math.random() * 255);
        int c2 = (int) (Math.random() * 255);
        int c3 = (int) (Math.random() * 255);

        // if rows is equal to columns then set color of square to red
        // in order to get a red diagonal from top left to bottom right
        // also if rows = 0 or length - 1 or columns = 0 or length - 1
        // then set square to red to make a red border
        if (row == 0 || col == 0 || row == ROWS - 1 || col == COLS - 1
        || row == col)
        {
            // make the border and diagonal all red
            c1 = 255;
            c2 = 0;
            c3 = 0;
        }

        // set the color at each specific point in board to respective RGB
        myColor[row][col] = new Color(c1, c2, c3);
        }
    }
}

@Override
public Dimension getPreferredSize() {
return new Dimension(COLS * BOX_SIZE, ROWS * BOX_SIZE);
}

@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();

int xOffset = (getWidth() - (COLS * BOX_SIZE)) / 2;
int yOffset = (getHeight() - (ROWS * BOX_SIZE)) / 2;


for (int row = 0; row < ROWS; row++) {
for (int col = 0; col < COLS; col++) {
int index = (row * COLS) + col;
g2d.setColor(myColor[row][col]);
g2d.fillRect(xOffset + (col * BOX_SIZE),
yOffset + (row * BOX_SIZE),
BOX_SIZE, BOX_SIZE);
}
}
g2d.dispose();
}

}

}