Color.Lerp does not seem to update for any apparent reason, any ideas?

I'm trying to lerp the color of my scene's ambient light from black to some color (I haven't decided what yet). for testing, I tried to get it to lerp to yellow, but it seems as if the Lerp function is only called once and never called again and I have no idea why. Here's my code, any help would be much appreciated.

 var startColor : Color = Color.black;
 var changeColor : Color = Color.black;
 var endColor : Color;
 var lerpedColor : Color;
 var changeTime : float = 5.0;
 
 function Start () {
     startColor = RenderSettings.ambientLight;
     endColor = RenderSettings.ambientLight;
     lerpedColor = startColor;
 }
 
 function Update () {
     lerpedColor = Color.Lerp(startColor, endColor, changeTime * Time.deltaTime);
     RenderSettings.ambientLight = lerpedColor;
 }
 
 function OnTriggerEnter (other : Collider) {
     endColor = changeColor;
 }
 
 function OnTriggerExit (other : Collider) {
     endColor = startColor;
 }