Bool based on objects existing not changing.

While doing a tutorial exercise called "Chaos Ball" from a book, one aspect of the code was not triggering a bool that displays a GUI element (and my code is almost exactly the same as both the printed and example files, as far as i can tell- the only thing I've changed is the initial bool being set to "false" because the printed "true" didn't work and doesn't make sense to me if you're testing for it to be positive).

I've tested things with print statements to monitor the state of the bool and have tried other if statement approaches as an attempted work around (which just end up instantly making the bool true and triggering the GUI message right away instead of after the conditions are met that it's supposed to wait for). So, as far as I can tell, it's either the bool itself or the way the script references objects in regards to the bool. I'm just curious to figure out what's going wrong here or what I'm not understanding about the shorthand code being used.

The tutorial project is several colored balls bouncing around an arena which get destroyed when they collide with a matching goal color. The destruction and movement is controlled by other scripts. This script is just monitoring the state of the game to trigger the GUI message when all four colored balls are destroyed. (all four ball objects from the Hierarchy are assigned to their slots in the script in the Inspector as well).

 using UnityEngine;
 using System.Collections;
 
 public class GameControlScript11 : MonoBehaviour 
 {
 
     public GameObject blueBall, greenBall, redBall, orangeBall;
     private bool isGameOver=false;
     
  
     void Update () 
     {
         isGameOver = !blueBall && !greenBall && !redBall && !orangeBall; 
     }
 
     void OnGUI()
     {
         if (isGameOver) 
         {
                //the (working) code for displaying the message using GUI.Box & GUI.Label  goes here
         }
     }
 
 
 }