Accessing component of a single prefabs' instance.

Hello everyone, I'm new to Unity and coding. My question may be simple, but I couldn't find a simple answer to this problem anywhere.

I'm trying to clone a prefab multiple times and set alpha (or color) of its every instance to random. My problem: after the Instantiaton all the clones look the same.

Code I attach to an empty game object (works fine):

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class Spawner : MonoBehaviour {
 
     public GameObject prefab;
     public Transform canvas;
     public int rowsInt;
     public int columnsInt;
 
 
     void Start()
     {
         StartCoroutine(SpawnDelay());
     }
         IEnumerator SpawnDelay()
     {
             for (int x = 0; x < rowsInt; x++)
             {
                 for (int y = 0; y < columnsInt; y++)
                 {
                     Instantiate(prefab, new Vector3(x,y), transform.rotation, canvas);
                     yield return new WaitForSeconds(0.2f);
                 }
             }
         }
     }

Code attached to the prefab:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class PrefabScript : MonoBehaviour
 {
     Image img;
     Color color;
 
     void Awake()
     {
         img = GetComponent<Image>();
 
         color = img.material.color;
         color.a = Random.Range(0.1f, 0.9f);
         img.material.color = color;
 
     }
 }

I guess I'm changing the component of the prefab and not the instances, but how can I get access to the components of the clones?