Random Material

I'm a beginner programmer, and im trying to make a city building game. I thought a good place to start would be placing objects, of which I did by making a raycast from the mouse, and instantiating the object at the hit.Point The first object I have tried to do this with are trees, however they have different materials, so light green, dark green, orange etc. I then created the following script in order to change the material when the tree is instantiated, however it wont work for some reason:

  using UnityEngine;
     using System.Collections;
     
     public class treeSettings : MonoBehaviour 
     {
         public Material[] myMaterials;
     
         void Start () 
         {
             gameObject.GetComponent<Renderer>().material = myMaterials[Random.Range(0,myMaterials.Length)];
         }
     }

I have assigned the materials to the list, but when I press play and click to instantiate the trees, it is only one colour.... what am I doing wrong? Thanks for any help :)