Ground cycle destroy,Ground destruction problem

Hi, I'm making a roll a ball game to find my level cuz I didn't used unity for a long.

I made a 15x15 ground separated in 15x15 (225) cubes.

I want one of them get selected every 5 sec then color it red for 0.5 sec and then destroy it.

My problem is that I want the cubes to disapear one by one evry 5 sec and they all disapear in a few seconds.

Youtube video showing what's happening.

Here's my script:

 #pragma strict
 
 var RN : int = -1;
 var groundPart : GameObject[];
 var destroyCooldown : int = 5;
 var timer : float = 0;
 var groundIsDestroyed : boolean = true;
 
 function Start () {
     groundPart = GameObject.FindGameObjectsWithTag("Ground");
 }
 
 function Update () {
     if(groundIsDestroyed == true) {
         Cooldown();
     }
 }
 
 function Cooldown () {
     yield WaitForSeconds(destroyCooldown);
     groundIsDestroyed = false;
     DestroyGround();
 }
 
 function DestroyGround () {
     if(groundIsDestroyed == false) {
     RN = Random.Range(0, groundPart.length);
     if(RN <= groundPart.length) {
         groundPart[RN].GetComponent(MeshRenderer).material.color = Color.red;
         yield WaitForSeconds(0.5);
         Destroy(groundPart[RN]);
     }
     RN = -1;
     groundIsDestroyed = true;
     }
 }

Thank you for ur help.