ExecuteInEditMode doesn't work on instantiated list of objects?

I have a prefab of a single tile, and for level design purposes I want to be able to make it 'invalid' and change it's colour to red both in play and edit modes. It has a bool checkbox in the inspector which allows me to do this.

I have made sure to include [ExecuteInEditMode] at the top of this script, and the below code WORKS JUST FINE in both play mode and edit mode.

 [SerializeField]
 private bool isBlocked;    

     private Renderer rend;                
 
     void Awake(){
         rend = gameObject.GetComponent<Renderer> ();
     }
     
     void Update(){
         DisplayTileValidity ();
     }
 
     void DisplayTileValidity(){
         if (isBlocked || isOccupied) {
             rend.sharedMaterial.color = Color.red;                        
         } else {
             rend.sharedMaterial.color = Color.white;
         }
     }

These tiles are prefabbed, and used as a gameobject to be instantiated in a grid... The code I am using is here, and WORKS FINE except for that I now cannot change the colour of the tile using that checkbox. It won't work in edit mode, or play mode.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class GridGenerator : MonoBehaviour {
 
     [Header("Grid Size")]
     [SerializeField]
     private int rowsAmount = 3;                            //The width of the level
 
     [SerializeField]
     private int columnsAmount = 3;                        //The length of the level
 
     [Header("Parents and Prefabs")]
     [SerializeField]
     private GameObject tilePrefab;                        //The tile prefab gameobject.
 
     [SerializeField]
     private Transform tileParent;                        //The Tile Parent gameobject for storing our tiles nicely in the hierarchy.
 
     private List<Vector3> gridPositions = new List<Vector3>();
 
     void Awake(){
         GenerateGrid ();
     }//end awake
 
     public void DefineGridPositions(List<Vector3> gridPositions){
 
         gridPositions.Clear ();                                        
 
         for (int x = 0; x < rowsAmount; x++) {                        
         for(int z = 0; z < columnsAmount; z++){                    
 
         Vector3 gridPosition = new Vector3 (x, 0, z);            
         gridPositions.Add (gridPosition);                    
 
             }//end for column
         }//end for row
 
     }//end define grid positions
 
     public void CreateGrid(List<Vector3> gridPositions){
 
         foreach(Vector3 gridPosition in gridPositions){
             GameObject tile = Instantiate (tilePrefab, gridPosition, Quaternion.identity) as GameObject;
             tile.transform.parent = tileParent;
         }//end foreach
 
     }//end create grid
 
     public void GenerateGrid(){
         DefineGridPositions (gridPositions);
         CreateGrid (gridPositions);
     }//end generate grid
 
 }

I assumed that by instantiating a whole list full of these tiles that I would simply be able to click whichever one I want in the hierarchy and make it red/ invalid, but that doesn't seem to be the case.

Do I have to add a script to the tile holder object or a function to the grid generator script to iterate over these tiles and change their material colour that way?

Please remember that the solution must work in both play and edit modes.

Thanks very much.