How do I reference a GameObject after finding its collider with Physics.OverlapSphere()?

Currently I have a Player game object placed in the scene with a script that generates TerrianCubes (just built in Cubes) underneath the player, places some other WorldFeatures (spheres) randomly around the scene, and then places TerrainCubes forming a path connecting that WorldFeature back to the player. It's rudimentary. The path isn't always player navigable, but it does what I want which is to start the scene with a bunch of Terrain Cubes connecting WorldFeatures back to the Player.

Now I'm trying to add a script to the TerrainCubes so that they instantiate with different colors. My goal is to have TerrainCubes with no neighbors start with a random color and for those with neighbors to derive their color from those neighbors. The code runs (I can play the Scene), but only a few blocks around the Player change color and I get this error msg ("NullReferenceException: Object reference not set to an instance of an object.")

Here's the DefineTerrainScript I'm using for that:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class DefineTerrainScript : MonoBehaviour {
 
     int rColor = 0,
         gColor = 0,
         bColor = 0,
         aColor = 0;
 
     Collider[] hitTerrainCubes = Physics.OverlapSphere(new Vector3(0.5f,0.5f, 0.5f), 1f);
 
     // Use this for initialization
     void Start () {
 
         if (hitTerrainCubes.Length == 0){
 
             rColor = Random.Range (0,3);
             gColor = Random.Range (0,3);
             bColor = Random.Range (0,3);
             aColor = Random.Range (0,3);
 
         }
         else {
 
             for (int i = 0; i < hitTerrainCubes.Length; i++){
 
                 GameObject hitTerrainCube = hitTerrainCubes[i].transform.gameObject;
 
                 rColor += hitTerrainCube.GetComponent<DefineTerrainScript>().rColor;
                 gColor += hitTerrainCube.GetComponent<DefineTerrainScript>().gColor;
                 bColor += hitTerrainCube.GetComponent<DefineTerrainScript>().bColor;
                 aColor += hitTerrainCube.GetComponent<DefineTerrainScript>().aColor;
 
             }
         
         }
 
         this.gameObject.renderer.material.color = new Color(rColor%4,
                                                             gColor%4,
                                                             bColor%4,
                                                             aColor%4);
 
     }

• I had some trouble finding the center and radius of the current TerrainCube to use for Physics.OverlapSphere so I just put in fixed numbers for now :(

My understanding is that the TerrainCubes will instantiate in order, so that later TerrainCubes will be able to see earlier ones. Here's my GenerateTerrainScript for reference:

     public GameObject terrainCube;
     public GameObject player;
     public float gridX;
     public float gridZ;
     public float spacing;
     public int startingFeatures;
     public GameObject worldFeatures;
     public float worldSize;
 
     // Use this for initialization
     void Start () {
 
         Vector3 playerStart = new Vector3(player.transform.position.x, player.transform.position.y, player.transform.position.z) * spacing;
         AddInitialTerrain(playerStart);
 
         for (int i = 0; i < startingFeatures; i++){
 
             Vector3 pos = (Vector3)PlaceWorldFeatures(worldFeatures);
             AddInitialTerrain(pos);
             ConnectAtoB(pos, playerStart);
 
         }
     
     }
     
 
     // Update is called once per frame
     void Update () {
 
         //SearchForThisHere(emptySpaces, player);
         //FillEmptySpace();
 
     }
 
 
     // Adds terrain around worldFeatures at start of new level
     private void AddInitialTerrain(Vector3 pos){
 
         pos.x -= (int)(gridX/2);
         pos.y--;
         pos.z -= (int)(gridX/2);
 
         for(int z = 0; z < gridZ; z++){
             
             for(int x = 0; x < gridX; x++){
 
                 Vector3 placeHere;
                 placeHere.x = pos.x + x;
                 placeHere.y = pos.y;
                 placeHere.z = pos.z + z;
                         
                 Instantiate(terrainCube, placeHere, Quaternion.identity);
                     
             }
         }
 
     }
 
     // Instantiates worldFeatures and returns position for adding terrain around.
     private Vector3? PlaceWorldFeatures(GameObject placeThis){
 
         Vector3 pos = new Vector3(Mathf.Round(Random.Range(-worldSize, worldSize)),
                                   Mathf.Round(Random.Range(-worldSize, worldSize)),
                                   Mathf.Round(Random.Range(-worldSize, worldSize))) * spacing;
         Instantiate(placeThis, pos, Quaternion.identity);
 
         return pos;
 
     }
 
     private void ConnectAtoB(Vector3 a, Vector3 b){
 
         Vector3 newB = new Vector3(b.x-1, b.y-1, b.z-1);
         while(a != newB){
 
             if(a.x < newB.x){a.x++;}
             if(a.x > newB.x){a.x--;}
 
             if(a.y < newB.y){a.y++;}
             if(a.y > newB.y){a.y--;}
 
             if(a.z < newB.z){a.z++;}
             if(a.z > newB.z){a.z--;}
 
             Instantiate(terrainCube, a, Quaternion.identity);
         }
     }

Comments about best practices, optimization, and alternative code are appreciated.