Script sets color to multiple objects from instead of only one object with Random.range()

have made a c# script that sets a color to my objects (in this case tiles). The problem is it sets a color to multiple objects instead of only one object. I have 16 Objects (tile) that are children of one empty gameobject (tiles). Every 1.5 sec (for example) I want only one tile to change color, randomly.

As well c# as javascript solutions are welcome.

 using UnityEngine;
 using System.Collections;
 
 public class TileManager : MonoBehaviour {
 
     private float timer = 0.0f;
 
     Color red = Color.red;
     Color blue = Color.blue;
     Color green = Color.green;
     //Color white = Color.white; FOR LATER USE
 
     public Transform tiles;
 
     void Update () 
     {        
         timer += Time.deltaTime;
 
         if (timer >= 1.5f)//change the float value here to change how long it takes to switch.
         {
             
             Transform tile = tiles.GetChild (Random.Range (0, 16));
 
             int rand = Random.Range (1, 4);
 
 
                 if (rand == 1) {
                     tile.GetComponent<Renderer> ().material.color = red;
                 } else if (rand == 2) {
                     tile.GetComponent<Renderer> ().material.color = blue;
                 } else {
                     tile.GetComponent<Renderer> ().material.color = green;
                 }
             timer = 0;
         }
     }
 }