Screen flashes red on damage?

So do I just create an empty game object and attach this script or do I use this method on the UI panel?

         if (getDamage)
         {
             Color Opaque = new Color(1, 1, 1, 1);
             bloodImage.color = Color.Lerp(bloodImage.color, Opaque, 20 * Time.deltaTime);
             if (bloodImage.color.a >= 0.8) //Almost Opaque, close enough
             {
                 getDamage = false;
             }
         }
         if (!getDamage)
         {
             Color Transparent = new Color(1, 1, 1, 0);
             bloodImage.color = Color.Lerp(bloodImage.color, Transparent, 20 * Time.deltaTime);
         }

just set getDamage to true when you get hit.

I think the easier way is to create a script to manage the red panel gameObject.

First you should create a new script; in my exemple I will call it DamagedEffect.

In your script create a public function who will change state of the panel gameObject (SetActive method) https://docs.unity3d.com/ScriptReference/GameObject.SetActive.html

Now you can create the panel in your scene : GameObject > UI > Panel Attache this gameObject to the DamagedEffect script.

Now you just need to call the function of DamagedEffect script in your main script ;)

I know this is an old question, but for people out there still looking for a possible solution using Coroutines and LeanTween:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class TestCameraFlashDamage : MonoBehaviour {
 
     public KeyCode activationKey;
     public Color targetColor;
     public float duration;
     private GameObject flashView;
 
     void Start () {
 
         // Create Flash Panel
         flashView = new GameObject ();
         flashView.name = "FlashCanvas";
         flashView.AddComponent<Canvas> ();
         Canvas myCanvas = flashView.GetComponent<Canvas> ();
         myCanvas.renderMode = RenderMode.ScreenSpaceOverlay;
         flashView.AddComponent<CanvasScaler> ();
         flashView.AddComponent<GraphicRaycaster> ();
         flashView.transform.SetParent (transform);
         GameObject panel = new GameObject ("Panel");
         panel.AddComponent<CanvasRenderer> ();
         panel.AddComponent<Image> ();
         RectTransform rt = panel.GetComponent<RectTransform> ();
         rt.sizeDelta = new Vector2 (2000, 2000);
         panel.transform.SetParent (myCanvas.transform, false);
         flashView.SetActive (false);
     }
 
     IEnumerator PlayAnimation () {
         Debug.Log ("Show Flash Damage Animation");
         
         if (!flashView.activeSelf) {
             flashView.SetActive (true);
         }
         Image img = flashView.GetComponentInChildren<Image>();
         LeanTween.value (flashView, 1.0f, 0.0f, duration)
             .setOnUpdate ((float val) => {
                 if (img != null) {
                     img.color = new Color (targetColor.r, targetColor.g, targetColor.b, val);
                 }
             })
             .setOnComplete (() => {
                 flashView.SetActive (false);
             });
         return null;
     }
 
     void Update () {
 
         if (Input.GetKeyUp (activationKey)) {
             StartCoroutine ("PlayAnimation");
         }
     }
 }