Completely transparent. RGBA is (0, 0, 0, 0).

	// clear the color of this transform's material
	transform.renderer.material.color = Color.clear;

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        transform.renderer.material.color = Color.clear;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		transform.renderer.material.color = Color.clear