Accessing a nested list of gameobjects not working quite right

Hi I am building a grid of tiles using the following code.

 Public GameObject tilePrefab;
 int mapLength = 10;
 int tileLength = 10;
 List<List<Tile>> map = new List<List<Tile>>();
 
 Awake()
 {
   MakeMap();
   ShowCenter();
 }
 
 
 void MakeMap()
 {
   List<Tile> row = new List<Tile>();
   for(int x = 0; x < mapLength;x++)
   {
      for(int z = 0; z < mapLength;z++)
      {
           Tile t = ((GameObject)Instantiate(tilePrefab,new Vector3(x * tileLength, 0, z * tileLength),new Quaternion.Euler(new Vector3())).GetComponent<Tile>();
           t.mapPosition = new Vector2(x,z);
           row.Add(t);
      }
      map.Add(row);
   }
 
 }
 
 void ShowCenter()
 {    
      //maplength will always be an odd number
      int center = (int)Mathf.Round(mapLength / 2);
      Tile c = map[(int)center][(int)center];
      c.transform.renderer.material.color = Color.red;
 }

The issue I am having is the ShowCenter method, I will only have the first column affected and show the tile at the correct location in one dimension only not both the x and z dimensions.

I hope I am explaining this well enough. below is the bare bones of my tile class which is attached to my Tile Prefab GameObject.

Any help is appreciated.

 public class Tile
 {
    public Vector2 mapPosition = Vector2.zero;
 
 }