Color with an alpha channel texture format.

	function Start () {
		// Create a new ARGB texture and assign it to the renderer's material
		var texture = new Texture2D (128, 128, TextureFormat.ARGB32, false);
		renderer.material.mainTexture = texture;
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Start() {
        Texture2D texture = new Texture2D(128, 128, TextureFormat.ARGB32, false);
        renderer.material.mainTexture = texture;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Start() as void:
		texture as Texture2D = Texture2D(128, 128, TextureFormat.ARGB32, false)
		renderer.material.mainTexture = texture