Need help with changing color of JOptionPane

Hello,

I need to change the color of output box to white. I guess the best would be to use UIManager.put("OptionPane.messageForeground", Color.white);? how do i use this? Sorry, I am very new to java

Code java:

 
import java.awt.Color;
 
import  javax.swing.*;
 
public class square{
 
 
 
	public void perimeter(){
 
 
		double sq;
 
	String shape2 = JOptionPane.showInputDialog("Enter shape you want to calculate the circumference for - Enter 1-square, 2-rectangle, 3-circle, 4-triangle, and 5-Exit.  ");
	int shape =  Integer.parseInt(shape2);
 
 
	 if (shape ==1){
 
 
	String square2 = JOptionPane.showInputDialog("Enter numeric value in centimeters ");
 
	double square =  Double.parseDouble(square2);
 
	 sq= 4 * square;
 
			     JOptionPane.showMessageDialog(null,
			     "You selected Square \n Length of each side= " + square + " cm \n Area=" + sq +"cm");
 
			    }
 
 
		if (shape ==5){
 
			System.exit(0);
		}}}