How to change transparency in a raw image using code?,How to change the alpha on UI Raw Image in code?

I been trying to change raw image transperancy not image transparency

JavaScript:

public var rawImg : UI.RawImage = null;
public var alpha : byte = 255;
public function Update ()
{
	var color : Color;
	color = Color32(0,0,0,alpha);
	if(rawImg)rawImg.color = Color(rawImg.color.r,rawImg.color.g,rawImg.color.b,color.a);
}

C#:

using UnityEngine;
using UnityEngine.UI;
public class RawImg : MonoBehaviour
{
	public RawImage rawImg = null;
	public byte alpha = 255;
	public void Update ()
	{
		Color color;
		color = new Color32(0,0,0,alpha);
		if(rawImg)rawImg.color = new Color(rawImg.color.r,rawImg.color.g,rawImg.color.b,color.a);
	}
}