Bloodscreen effect

I’m trying to make a blood screen effect similar to that of CoD or most FPS games.
I’ve tried using the script from blood damage like call of duty?

But It doesn’t seem to show the texture when reaching 60% health. There are no errors.

using UnityEngine;
using System.Collections;

public class HealthScript : MonoBehaviour {
	public float HP = 100f;
	public float maxHP = 100f;
	public float healSpeed = 1f;
	
	public GUITexture bloodGui;

	void Start(){ 
	  Rect r = new Rect(-Screen.width/2, -Screen.height/2, Screen.width, Screen.height);
	  bloodGui.pixelInset = r;
	}
	
	float BloodAlpha(float start){
	    float alpha = 1 - (100 * HP)/(start * maxHP);
	    return Mathf.Clamp(alpha, 0, 1);
	}
	
	void Update(){
 	 Color temp;
 	 temp = bloodGui.color;
 	 temp.a = BloodAlpha(60);
  	 bloodGui.color = temp;
	}
}

temp must equal to GUI.color - you need to save normal color of GUI and restore it then.
Also you have BloodAlpha defined as variable, not as function. Use

void BloodAlpha (float start){