Random colors?

Ok I have a script which is run to give 4 different balls random colors that I have chosen. The problem is that I don't want any of the balls to be the same color but they are. How do I fix this? Here is script:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class Everything : MonoBehaviour { public string currentColor; public SpriteRenderer sr; public SpriteRenderer sr1; public SpriteRenderer sr2; public SpriteRenderer sr3; public SpriteRenderer sr4; public Color colorGreen; public Color colorRed; public Color colorBlue; public Color colorYellow;

 // Use this for initialization

 void Start () {
     SetColor ();
 }
 
 // Update is called once per frame
 void Update () {
     
 }

 void SetColor () {
     int index = Random.Range (0, 3);
     switch (index) {
     case 0:
         currentColor = "Green";
         sr.color = colorGreen;
         break;

     case 1:
         currentColor = "Red";
         sr.color = colorRed;
         break;



     case 2:
         currentColor = "Blue";
         sr.color = colorBlue;
         break;



     case 3:
         currentColor = "Yellow";
         sr.color = colorYellow;
         break;
     }

     int i = Random.Range (0, 3);
     switch (i) {
     case 0:
         currentColor = "Green";
         sr1.color = colorGreen;
         break;

     case 1:
         currentColor = "Red";
         sr1.color = colorRed;
         break;



     case 2:
         currentColor = "Blue";
         sr1.color = colorBlue;
         break;



     case 3:
         currentColor = "Yellow";
         sr1.color = colorYellow;
         break;
     }

     int ind = Random.Range (0, 3);
     switch (ind) {
     case 0:
         currentColor = "Green";
         sr2.color = colorGreen;
         break;

     case 1:
         currentColor = "Red";
         sr2.color = colorRed;
         break;



     case 2:
         currentColor = "Blue";
         sr2.color = colorBlue;
         break;



     case 3:
         currentColor = "Yellow";
         sr2.color = colorYellow;
         break;
     }

     int wow = Random.Range (0, 3);
     switch (wow) {
     case 0:
         currentColor = "Green";
         sr3.color = colorGreen;
         break;

     case 1:
         currentColor = "Red";
         sr3.color = colorRed;
         break;



     case 2:
         currentColor = "Blue";
         sr3.color = colorBlue;
         break;



     case 3:
         currentColor = "Yellow";
         sr3.color = colorYellow;
         break;
     }

     int cool = Random.Range (0, 3);
     switch (cool) {
     case 0:
         currentColor = "Green";
         sr4.color = colorGreen;
         break;

     case 1:
         currentColor = "Red";
         sr4.color = colorRed;
         break;



     case 2:
         currentColor = "Blue";
         sr4.color = colorBlue;
         break;



     case 3:
         currentColor = "Yellow";
         sr4.color = colorYellow;
         break;
     }

 }