Confused about a script that is changing a gameobjects material colour but not sure what the issue is.

Hello there! I am currently trying to work out why this particular script isn't working as I'd hoped. In my little game I am trying to make it so that when an enemy spawns on a tile, it'll turn red but for some reason it isn't working.

 private void Update()
 {
     ChangeTileMaterial();
 }

 public void NextWave()
 {
     m_currentWaveIndex++;
     //Check the next wave exists
     if(m_currentWaveIndex - 1 < m_waves.Length)
     {
         m_currentWave = m_waves[m_currentWaveIndex - 1];
         m_enemiesToSpawn = m_currentWave.m_numberOfEnemies;
         StartCoroutine(Spawn());
     }

 }

 IEnumerator Spawn()
 {
     while(m_enemiesToSpawn > 0)
     {
         if (Time.time > m_nextSpawnTime)
         {
             Transform spawnTile = m_map.getRandomOpenTile();
             Material tileMaterial = spawnTile.gameObject.GetComponent<Renderer>().material;
             tileMaterial.color = Color.red; //Change color
             m_nextSpawnTime = Time.time + m_currentWave.m_timeBetweenSpawn;
             Enemy newEnemy = Instantiate(m_enemy, spawnTile.position, Quaternion.identity) as Enemy;
             newEnemy.transform.parent = m_enemyHolder;
             m_enemiesToSpawn--;
         }

         if (m_enemiesToSpawn <= 0)
         {
             NextWave();
         }

     yield return new WaitForSeconds(m_currentWave.m_timeBetweenSpawn);
     }
 }

 private void ChangeTileMaterial()
 {
     Transform spawnTile = m_map.getRandomOpenTile();
     Material tileMaterial = spawnTile.gameObject.GetComponent<Renderer>().material;

     tileMaterial.color = Color.red;
 }

When calling the color change by the coroutine it isn't working but when calling it with the ChangeTileMaterial() function within update it does work. I have tried calling the coroutine from Update and this doesn't work. Does anyone have any ideas?

The coloration change seems to happen at random intervals. On every 6th spawn of an enemy the colour will change of that tile. I set my enemy spawn count to 15 for example and the colour will change on 5, 8 and 13 of an enemy spawn. http://imgur.com/EwJnGwF

Thank you in advance.