Color.r  

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

using UnityEngine;
using System.Collections;

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

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public color as Color = Color.white

def Awake():
color.r = 0