TextureFormat.Alpha8  

function Start () {
// Create a new alpha-only texture and assign it
// to the renderer's material
var texture = new Texture2D (128, 128, TextureFormat.Alpha8, false);
renderer.material.mainTexture = texture;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Start() {
Texture2D texture = new Texture2D(128, 128, TextureFormat.Alpha8, false);
renderer.material.mainTexture = texture;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Start():
texture as Texture2D = Texture2D(128, 128, TextureFormat.Alpha8, false)
renderer.material.mainTexture = texture