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