Where can a Lerp be used?

I'm trying to make a lerp for one of my enemies once its trigger is activated. So I have to put it in the function that dictates that, which is a collision function.

Does lerp only work in function Update? Or do I have something wrong?

Here are the relevant parts of my script:

 var thePrefab: GameObject;
 var enemyActive : boolean = true;
 var jelReg : Material;
 var jelShock : Material;
 var duration = 60.0;
 
 private var coin : GameObject;
 private var player : PlayerScript;
 
 function Start(){
 player = GameObject.FindWithTag("Player").GetComponent(PlayerScript);
 renderer.material = jelReg;
 }
 
 function Update () {
 }
 
 
 function OnCollisionEnter(theCollision : Collision){
     if(player.Diamond == true){
         Destroy(gameObject.transform.root.gameObject);
         coin = Instantiate(thePrefab, transform.position, Quaternion.identity);
     }
     if(player.Diamond == false){
         if(theCollision.gameObject.tag == "Player"){
             yield WaitForSeconds(2);
             gameObject.collider.isTrigger = true;
             enemyActive = true;



             var lerp : float = Mathf.PingPong (Time.time, duration) / duration;
             renderer.material.Lerp (jelReg, jelShock, lerp);




             yield WaitForSeconds(3);
             gameObject.collider.isTrigger = false;
             renderer.material = jelReg;
             enemyActive = false;
         }
     }
 }

it's probably something small, or that lerps just can't be used outside of the update function. Whatever the case, please help!