How to change the alpha of a sprite

I have a script and I want it so that when I cant press a button the the alpha changes to half. my script is:

using UnityEngine;
using System.Collections;

public class Play : MonoBehaviour {

	public static bool readyToGo;
	public SpriteRenderer me;

	void OnMouseDown(){
		if (readyToGo) {
			if (SelectMode.modeSelect == 0) {
				Application.LoadLevel ("Original");		
			}
			if (SelectMode.modeSelect == 1) {
				Application.LoadLevel ("Ascension");		
			}
			if (SelectMode.modeSelect == 2) {
				Application.LoadLevel ("Freefall");		
			}
		}
	}

	void Update(){
		if (!readyToGo) {
			//me.sprite.a = 0.5f;		
		}
	}
}

Does anybody know how to achieve this?

In the class:

private Color c = new Color(1F,1F,1F,0.5F);

Where you need to set its alpha:

me.color = c;