How do I make my healthbar change color depending on damage?

Hello, I'm really stuck on my healthbar and could really use some help! I'm trying to make my healthbar go from green -> yellow -> red depending on how much damage the player has gotten. But my current attempt at a color change code in my setHealth() {...} just makes the healthbar vanish :/

Any ideas on what I can do to make this work? Because I'm really lost :( THANKS!

PS. I've commented out the color code part on purpose!

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class EnemyHealth : MonoBehaviour {
     
     public float hitPoints = 100f;
     public float currentHitPoints;
     public GameObject destroyFX;
     public Image healthbar;
 
     public Color32 startColor;
     public Color32 middleColor;
     public Color32 endColor;
 
     void Start () {
         currentHitPoints = hitPoints;
         setHealth();
     }
     
     void TakeDamage(float amt) { 
         currentHitPoints -= amt;
 
 
         if (currentHitPoints <=0) {
             currentHitPoints = 0;
             Die();
         }
 
         setHealth();
 
     }
     
     void Die() {
         if(gameObject.tag == "Enemy") {
             Instantiate(destroyFX, this.transform.position, this.transform.rotation); 
             Destroy (gameObject);
         }
     }
 
     public void setHealth() {
         float enemyHealthCalc = currentHitPoints / hitPoints; 
 
         healthbar.transform.localScale = new Vector3(enemyHealthCalc, healthbar.transform.localScale.y, healthbar.transform.localScale.z);
 
         /*
         if (enemyHealthCalc < 0.1f) {
             healthbar.color = Color.Lerp(endColor, middleColor, enemyHealthCalc * 2);
         }
         else {
             healthbar.color = Color.Lerp(middleColor, startColor, (enemyHealthCalc - 0.5f) * 2);
         }
         */
     }
 
 }