How do I swap out a gui texture button on mouse down

I can’t seem to find a proper way of swapping out my gui button on mouse down.
I basically want that when you press the button it will have an on mouse down appearance
So far my code is like this.

var DTMFSound : AudioClip;

function OnMouseDown(){
     yield WaitForSeconds (0.3);
     //Do something when button clicked


Its all in there

I had some old UNITY tutorials kicking around and found my own answer. This works perfectly but I could not find a working sample of this anywhere.

var mouseUpTexture : Texture2D;
var mouseDownTexture : Texture2D;
var beep : AudioClip;

function OnMouseDown(){
     guiTexture.texture = mouseDownTexture;

function OnMouseUp(){
     guiTexture.texture = mouseUpTexture;
     yield new WaitForSeconds(0.5);