var color : Color = Color.white;
color.a = 0;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Color color = Color.white;
    void Example() {
        color.a = 0;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public color as Color = Color.white

    def Example():
        color.a = 0