Change material of a single prefab

I want to change the material of a prefab every time damage is equal 2, however this changes every single instance of the prefab.

This the script in the prefab

 public Material[] materials;
 public Renderer rend;

 void start()
     {
 
         rend = GetComponent<Renderer>();
         rend.enabled = true;
 
         damage = 1;
     }
  void Update()
   {
         if (damage == 2)
         {
             rend.material = materials[0];
         }
         
 }

this one is the player's script that instantiate the prefabs.

 public Rigidbody ball;
 
  private void Shotter()
         {
             if (Input.GetMouseButton(0) && ballSpeed< 50.0f)
             { 
                 ballSpeed += 25.0f * Time.deltaTime;
                 //Debug.Log(ballSpeed);
                 filler.fillAmount += 0.5f  * Time.deltaTime;
                 
             }
 
             if (Input.GetMouseButtonUp(0) && ballNumber < ballNumberLimit)
             {
 
                 Rigidbody ballInstance;
                 ballInstance = Instantiate(ball, barrelEnd.position, barrelEnd.rotation) as Rigidbody;
                 cubes.Add(ballInstance);
                 ballInstance.velocity = barrelEnd.forward * ballSpeed;
                 ballSpeed = 0.0f;
                 filler.fillAmount = 0.0f;
                 ballNumber += 1;
 
                
 
             }
         }