TextureFormat.Alpha8

using UnityEngine;
using System.Collections;

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

function Start () {
	// Create a new alpha-only texture and assign it
	// to the renderer's material
	//创建一个新的仅alpha纹理格式，并指定个渲染器材质
	var texture = new Texture2D (128, 128, TextureFormat.Alpha8, false);
	renderer.material.mainTexture = texture;
}