Change Transparency of a Cut-out Material

So I made a simple rectangle as a .png and imported it into unity. I placed it on a plane with the transparency-cut-out-specular material. I want the rectangle to fade out via opacity. I've been searching all around but I can't find a straight forward answer. The current script is:

 var colorStart : Color = Color.red;

 var colorEnd : Color = Color.green;

 var duration : float = 1.0;

 function Update () {

     var lerp : float = Mathf.PingPong (Time.time, duration) / duration;

     renderer.material.color = Color.Lerp (colorStart, colorEnd, lerp);
 }

I tried using: var colorEnd : Color = Color.clear; but that didn't work. Can you please help me. ][1]