type ...Material[] vs Material (materials change at run time)

I am trying to look through an array of materials on a specific game object and change the material at run time. I have read over multiple pages of the scripting reference on materials, game objects, render, transforms, Material.SetTexture and so on. I have also searched and read others answers on similar questions in the forums. All this research points to the fact that I should be using material[] to form an array which will allow me to set the material element I need to change.

However using the code below I get this: error CS0029: Cannot implicitly convert type UnityEngine.Material' to UnityEngine.Material[]'

I can not find a resolution for this error and I'm not sure how to convert the material to an array.

What am I doing wrong with my code?

 using UnityEngine;
 using System.Collections;
 
 public class ChangeMaterial : MonoBehaviour {
     
     //Put the game object with the texture you are changing here.
     GameObject Model;
     public string target;
     // put the first material here.
     public Material material1;
     // put the second material here.
     public Material material2;
     
     bool FirstMaterial = true;
     bool SecondndMaterial = false;    
     Material[] themes;
 
     
     void Start ()
     {
         
     Model = GameObject.Find (target);
     themes = Model.renderer.materials[4];
     themes[4] = material1;
     Model.renderer.materials[4] = themes; 
 
     }
      
     void OnMouseDown()
     {
         if (!SecondndMaterial)
             {
            themes[4] = material2;
            SecondndMaterial = true;
         FirstMaterial = false;
         }
      
         else if (SecondndMaterial)
         {
         themes[4] = material1;
         FirstMaterial = true;
         SecondndMaterial = false;
         }
     }
 }