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 Example() {
        transform.renderer.material.color = Color.gray;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

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