simple script, use it with LookCast.cs

LookColorChange.cs

using UnityEngine;
using System.Collections;

// put this script on an object with a COLLIDER
public class LookColorChange : MonoBehaviour {

	// ALL CODE IN "ONLOOK" WILL HAPPEN EVERY FRAME THIS THING IS LOOKED AT
	void OnLook () {
		// if we look in console, we should see this message
		Debug.Log ("this thing is getting looked at!!!");

		// =================================================
		// SAMPLE EFFECTS YOU COULD DO...
		// =================================================

		// grow in size
		transform.localScale += Vector3.one * Time.deltaTime;
		// grow in size 2 times faster
		transform.localScale += Vector3.one * Time.deltaTime * 2.0f;


		// change color of object to 100% red
		// (if no renderer, it will crash)
		renderer.material.color = new Color(1.0f, 0f, 0f);


		// turn on a Light (if there is no light, code will crash)
		light.enabled = true;
		// make a light increase in intensity as we look at it
		light.intensity += Time.deltaTime;


		// if not already playing a sound, then play any sound (AudioSource) on this object; if no AudioSource, will crash
		if (!audio.isPlaying) { audio.Play (); }
		// increase volume on any already-playing sound, as we look at it
		audio.volume += Time.deltaTime;


		// make physics object fly up if we look at it; will crash if there is no Rigidbody attached
		rigidbody.AddForce( Vector3.up * Time.deltaTime * 100.0f );
	}


}