Problem getting prefab color

I want to get the color of a prefab.

So I wrote this:

using UnityEngine; using System.Collections;

public class Test : MonoBehaviour {

 public GameObject prefab;
 
 void Update () {
     Color color = prefab.renderer.material.color;
     print(color);
 }

}

I attached this script to a gameObject in the scene and draged and droped a prefab with material on it on the "prefab" member variable in inspector.

Then I hit run i works, nice.

Then I hit stop. And my prefab material is gone, I must attach my material again everytime, I dont want that.

What am i doing wrong here? Is this a bug in Unity?

Thanks...