How would I randomize the materials of an object using this script.

Try this. I added a List of materials you will need to add to in the inspector. Enjoy.

using UnityEngine; using System.Collections.Generic;

 public class SkylineManager1 : MonoBehaviour {
  
     public Transform prefab;
     public int numberOfObjects;
     public float recycleOffset;
     public Vector3 minSize, maxSize;
     public List<Material> randomMaterials; //ADD THIS - and assign some materials in the inspector
  
     private Vector3 nextPosition;
     private Queue<Transform> objectQueue;
  
     void Start () {
        GameEventManager.GameStart += GameStart;
        GameEventManager.GameOver += GameOver;
        objectQueue = new Queue<Transform>(numberOfObjects);
        for(int i = 0; i < numberOfObjects; i++){
          objectQueue.Enqueue((Transform)Instantiate
                              (prefab, new Vector3(0f, 0f, -100f), Quaternion.identity));
        }
        enabled = false;
  
     }
  
     void Update () {
        if(objectQueue.Peek().localPosition.x + recycleOffset < Runner1.distanceTraveled){
          Recycle();
        }
     }
  
     private void Recycle () {
        Vector3 scale = new Vector3(
          Random.Range(minSize.x, maxSize.x),
          Random.Range(minSize.y, maxSize.y),
          Random.Range(minSize.z, maxSize.z));
  
        Vector3 position = nextPosition;
        position.x += scale.x * 0.5f;
        position.y += scale.y * 0.5f;
 
      
        
  
        Transform o = objectQueue.Dequeue();
        o.localScale = scale;
        o.localPosition = position;
        nextPosition.x += scale.x;
 
        //Random Material TIME!!!!
        if(randomMaterials != null && randomMaterials.Count > 0)
             o.gameObject.renderer.material = randomMaterials[Random.Range(0, randomMaterials.Count)];
 
 
        objectQueue.Enqueue(o);
     }
     private void GameStart () {
        nextPosition = transform.localPosition;
        for(int i = 0; i < numberOfObjects; i++){
          Recycle();
        }
        enabled = true;
     }
  
     private void GameOver () {
        enabled = false;
     }
 }