Color.operator /  

// white / 2 = gray
var grayColor : Color = Color.white / 2;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Color grayColor = Color.white / 2;
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public grayColor as Color = (Color.white / 2)