Random colors grid

What I should get in the end is a grid with squares that have stable colors if I resize the window. So far I get a grid that has random colors but as the everything is redrawn the colors are too. I'm thinking, maybe an array that stores the colors could work but I don't really know how to implement it in what I have so far.

public class GridRandomColors extends JFrame {


private static class Board extends JPanel {
    private Rectangle MAIN_RECT;
    private double BRICK_WIDTH, BRICK_HEIGHT;
    private int COLS = 8;
    private int ROWS = 8;


    public Board() {
        setBackground(Color.gray);
    }

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

    private void drawRectangle(Graphics g) {

        if (getHeight() > getWidth()) {

            MAIN_RECT = new Rectangle(0, 0, getWidth(), getWidth());
            g.fillRect(0, (getHeight()-getWidth())/2, getWidth(), getWidth());

            x = 0;
            y = (getHeight()-getWidth())/2;
        } else {
            x = (getWidth()-getHeight())/2;
            y = 0;
            MAIN_RECT = new Rectangle(0, 0, getHeight(), getHeight());
            g.fillRect((getWidth()-getHeight())/2, 0, getHeight(), getHeight());

        }
        BRICK_WIDTH = (float) MAIN_RECT.getWidth() / COLS;
        BRICK_HEIGHT = (float) MAIN_RECT.getHeight() / ROWS  ;
    }

    double spacing = 0.2;
    private double x;
    private double y;
    private Color color;

    private void drawBricks(Graphics g) {
        Graphics2D brick = (Graphics2D) g.create();

        for (int j = 0; j < ROWS; j++) {
            for (int a = 0; a < COLS; a++) {
                Random rand = new Random();
                color = new Color(rand.nextFloat(), rand.nextFloat(), rand.nextFloat());
                Color oldColor = g.getColor();
                brick.setColor(color);

                Rectangle2D.Double rect = new Rectangle2D.Double(x, y, BRICK_WIDTH - spacing*(COLS-1), BRICK_HEIGHT- spacing*(ROWS-1));

                brick.fill(rect);
                brick.setColor(oldColor);
                    x += BRICK_HEIGHT+spacing;
                }
            if (getHeight() > getWidth()) {
                x = 0;
            }
            else {
                x = (getWidth() - getHeight()) / 2;
            }

            y += BRICK_HEIGHT+spacing;
            }

        }
}



public GridRandomColors() {

    setDefaultCloseOperation(EXIT_ON_CLOSE);    //mai bine cu exit on close
    setSize(800, 820);
    add(new Board());
}

public static void main(String[] args) {
    java.awt.EventQueue.invokeLater(new Runnable() {

        @Override
        public void run() {
            new GridRandomColors().setVisible(true);
        }
    });
}

}