gistfile1.cs

using UnityEngine;
using System.Collections;

public class Script : MonoBehaviour
{
	public int w = 100;
	private Texture2D tex;

	void Update ()
	{
		tex = new Texture2D (w, w);
		Color[] cs = new Color[w * w];
		for (int i=0; i<w*w; i++) {
			cs [i] = Color.gray * Mathf.PerlinNoise (Random.Range (0f, 1f), Random.Range (0f, 1f));
		}
		tex.SetPixels (cs);
		tex.Apply ();
	}

	void OnGUI ()
	{
		GUI.DrawTexture (new Rect (0, 0, Screen.width, Screen.height), tex);
	}
}