Just came across this problem myself, many of the answers were old so will update with my solution that works

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


public class Mat_library2 : MonoBehaviour
{
[Header("Materials")]
public Material[] myMaterials;
private Material newMaterial;
public GameObject elypseTile;
public int RequiredColorIndex;
public void Start()
{
    findAllMaterials();
}

public void findAllMaterials()
{
    //find all materials in resources folder Assets>Resources>Materials>
    myMaterials = Resources.LoadAll("Materials", 
    typeof(Material)).Cast<Material>().ToArray();
    Debug.Log("Materials " + 
    Resources.FindObjectsOfTypeAll(typeof(Material)).Length);
   
    foreach (Material mat in myMaterials)
    {
        Debug.Log("Material is: " + mat);
        Debug.Log("RequiredColorIndex: " + RequiredColorIndex);
    }
    //Apply material from indes to game object
    elypseTile.GetComponent<Renderer>().material.color = 
    myMaterials[RequiredColorIndex].color;
}

}

Just came across this problem myself, many of the answers were old so will update with my solution that works

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


public class Mat_library2 : MonoBehaviour
{
  [Header("Materials")]
  public Material[] myMaterials;
  private Material newMaterial;
  public GameObject elypseTile;
  public int RequiredColorIndex;
  
  public void Start()
  {
    findAllMaterials();
  }

  public void findAllMaterials()
  {
    //find all materials in resources folder Assets>Resources>Materials>
    myMaterials = Resources.LoadAll("Materials", 
    typeof(Material)).Cast<Material>().ToArray();
    Debug.Log("Materials " + 
    Resources.FindObjectsOfTypeAll(typeof(Material)).Length);
   
    foreach (Material mat in myMaterials)
    {
      Debug.Log("Material is: " + mat);
      Debug.Log("RequiredColorIndex: " + RequiredColorIndex);
    } 

    //Apply material from indes to game object
    elypseTile.GetComponent<Renderer>().material.color = 
    myMaterials[RequiredColorIndex].color;
  }
}