Randomize texture within prefab

Hello everyone, I decided to try and make a simple memory game where you match the cards. I have not used Unity or done any scripting since college but I thought I would try anyway. So I found a tutorial on youtube and have been following along, however I decided to create my own assets instead of using the ones they provided, and now I am stuck.

In part of the tutorial, they write the script that will randomize the texture on the card...My problem is that my 'card' is not one single object/model. It is a cube made up of quads ( I am trying to do this quickly so I didn't have time to uv map my images to a normal cube and had to use quads so I could have a bottom and top, etc)

So what needs to happen is I somehow create a line of code that will tell Unity to randomize the texture on only the 'Top' portion of my quad-cube prefab. I don't know how to tell the script to go inside the prefab and change just one part of it.

This is a screen cap of my workspace, showing the particular quad that I want to have a randomized texture: http://iron-zing.deviantart.com/art/Screen-Shot-2014-06-19-at-4-55-39-PM-462182228

And here are a few of my current scripts:

This one is attached to the playmat:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 public class Playmat : MonoBehaviour 
 {
     public GameObject cardPrefab;
     public Layout layout;
     public GameObject board;
 
     public List<Card>cards=new List<Card>();
 
     // Use this for initialization
     void Start () 
     {
         GameSettings.Instance ().SetDifficulty (GameSettings.GameDifficulty.Medium);
         CreateLayout ();
         CreateCardsFromLayout ();
         CreateCardTypes ();
     }
 
     void CreateLayout()
     {
         board = layout.GetRandomLayout ();
     }
 
     void CreateCardsFromLayout()
     {
         foreach (Slot s in layout.slots) 
         {
             GameObject go = GameObject.Instantiate(cardPrefab,s.transform.position,Quaternion.Euler(180,-180,360)) as GameObject;
             go.transform.parent = s.transform;
             Destroy (s.GetComponent<BoxCollider>());
             cards.Add (go.GetComponent<Card>());
         }
     }
 
     void CreateCardTypes()
     {
         for(int i=0;i< cards.Count/2;i++)
         {
             Card c1 = cards[i];
             Card c2 = cards[cards.Count/2 - 1 - i];
             string type = GameSettings.Instance ().GetRandomType ();
             c1.GenerateCard(type);
             c2.GenerateCard(type);
         }
     }
 
 
     // Update is called once per frame
     void Update () {
     
     }

And this one is attached to the card prefab

 using UnityEngine;
 using System.Collections;
 
 public class Card : MonoBehaviour 
 {
     public string CardType;
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     public void GenerateCard(string cardType)
     {
         CardType = cardType;
         renderer.material.mainTexture = Resources.Load ("Graphics/" + CardType) as Texture2D;
     }
 
 
 void OnMouseDown()
     {
         print ("I'm Clicked");
         }
 
 }

I'm sure it's a simple solution but I cannot figure it out for the life of me. If I could get the images on the cards to be random then the game would be working fine. This is the only problem I seem to be having with it. Please let me know if you can help out, thank you!