Need help with 2D Arrays (problems with navigating)

Hello everyone! Sorry for my bad English :)

I'm trying to understand logic of 2d arrays and make a grid for a turn-based game. After making an array and generating prefabs, no tiles are highlighted with red. What can I do to highlight a tile I want? This tile must be the tile right in the middle of my map. My code:

     #pragma strict
 var tiles : GameObject[,] = new GameObject [32,32];
 var TilePrefab : GameObject;
 var x : int = 32;
 var z : int = 32;
 function Start () {
     for (var i = 0; i < z; i++)
         for (var j = 0; j < x; j++) {
             Instantiate (TilePrefab, Vector3(j*10, 0, i*10), Quaternion.identity);
             tiles[j,i] = TilePrefab;
         }
             tiles[16,16].renderer.material.color = Color.red;
             print("It is working");
 }