1getContentPane().setBackground(Color.YELLOW);  //Whatever color

1package Exaple;
2
3import java.awt.Color;
4
5import javax.swing.JFrame;
6
7public class Window {
8
9	public static void main(String[] args) {
10		
11		JFrame frame = new JFrame(); 
12		frame.setExtendedState(JFrame.MAXIMIZED_BOTH); 
13		frame.setVisible(true);
14		frame.setTitle("Expaple");
15		frame.setBackground(Color.BLUE);
16	}
17
18}

1setBackground(Color.red);