HealthBar.cs

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

public class HealthBar : MonoBehaviour
{

    public PlayerHealth playerHealth;
    public Image fillImage;
    private Slider slider;
    public float fillValue;


    // Start is called before the first frame update
    void Awake()
    {
        slider = GetComponent<Slider>();
    }

    // Update is called once per frame
    void Update()
    {
        if(slider.value <= slider.minValue)
        {
            fillImage.enabled = false;
        }

        if(slider.value > slider.minValue)
        {
            fillImage.enabled = true;
        }

        if(fillValue <= slider.maxValue / 3)
        {
            fillImage.color = Color.red;
        }

        else if(fillValue > slider.maxValue / 3)
        {
            fillImage.color = Color.green;
        }

        fillValue = playerHealth.health / playerHealth.maxHealth;
        slider.value = fillValue;
    }
}