（スクリプト）

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

public class TankHP_2 : MonoBehaviour
{
    public GameObject effectPrefab;
    public AudioClip sound;

    private int HP = 50;
    public Text HPLabel;

    private void Start()
    {
        HPLabel.text = "HP:" + HP;
        HPLabel.color = Color.green;
    }

    private void OnCollisionEnter(Collision collision)
    {
        if (collision.gameObject.CompareTag("EnemyShell"))
        {
            HP -= 1;

            if(HP > 40)
            {
                HPLabel.color = Color.green;
            }
            else if(HP > 30)
            {
                HPLabel.color = Color.yellow;
            }
            else if(HP > 10)
            {
                HPLabel.color = Color.blue;
            }
            else
            {
                HPLabel.color = Color.red;
            }

            HPLabel.text = "HP:" + HP;

            Destroy(collision.gameObject);
            GameObject effect = Instantiate(effectPrefab, collision.transform.position, Quaternion.identity);
            Destroy(effect, 0.5f);
            AudioSource.PlayClipAtPoint(sound, transform.position);
        }
    }
}

（結果）

• HPの数に応じて、HPテキストの色が変化すれば成功