Change image's alpha via script... i have some problem

[Cervelx]

Hi guys! I’ve this image in my scene…

i want to change the alpha in a way like this…

     public Image MyImage;
     public bool imageBool;

vois Start()
    {
alphaImage = MyImage.GetComponent<SpriteRenderer>().color;
    }

void Update()
     {
     if (imageBool == true)
     {
     alphaImage .a -= 10;
     }
     }

But it not work for some reason… i have many red point in the ispector… there is a better way ? Thank you

[Masterio]

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

public class AlphaTest : MonoBehaviour 
{
	public SpriteRenderer sprite;
	public bool hide;
	public float alphaSpeed = 1f;

	private float _alpha = 1f;

	void Update()
	{
		if (hide)
		{
			if(_alpha > 0f)
			{
				_alpha = Mathf.MoveTowards(_alpha, 0f, Time.deltaTime * alphaSpeed);
				sprite.color = new Color(sprite.color.r, sprite.color.g, sprite.color.b, _alpha);
			}
		}
		else
		{
			if(_alpha < 1f)
			{
				_alpha = Mathf.MoveTowards(_alpha, 1f, Time.deltaTime * alphaSpeed);
				sprite.color = new Color(sprite.color.r, sprite.color.g, sprite.color.b, _alpha);
			}
		}
	}
}