How do you change the background of a JFrame?

[NightOwls]

As the title states, I can't get the background of my JFrame window to change.
I would've bet that just .setBackground(Color.RED) would've worked but no luck. Here's my code:

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

public class win {

    public static void main(String[] args) {

    	 JFrame guiWindow = new JFrame("Calculator");
         guiWindow.setBackground(Color.RED);   // <---- Nope :( 
         guiWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         guiWindow.setSize(260, 253);
         guiWindow.setLocation(200, 130);
         guiWindow.setVisible(true);

    }
}

If someone can tell me what I'm doing wrong or point me in the right direction it would be appreciated. Thanks.

[NightOwls]

Nevermind.

guiWindow.getContentPane().setBackground(Color.RED);