Color.gray  

// Assign a gray color to this transform's material
transform.renderer.material.color = Color.gray;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
transform.renderer.material.color = Color.gray;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
transform.renderer.material.color = Color.gray