GUI Testure Button Swap on MouseDown

I have this script I'm trying to get to work for an oxygen timer, it works fine but I'm trying to get a series of two Texture GUI buttons to swap out a different button texture on mouse down and swap back on mouseUp, but I'm not having a lot of luck figuring it out. I have it where the 2nd button state stays which is not what I want. here is my somewhat messy code... lol slowly getting messier the more I muck about with it.

  var beep1 : AudioClip;
  var beep2 : AudioClip;
  var clockIsPaused : boolean = false;
  var startTime : float; // (in sec)
  var timeRemaining : float; // (in sec)
  var percent : float;
  var clockBG : Texture2D;
  var clockFG : Texture2D;
  var buttonTexture: Texture2D;
  var miniButtonTexture : Texture2D;
  var miniButtonTexture2 : Texture2D;
  var clockBGMaxWidth : float; // the starting width of the foreground bar
  var TextureButton1: Texture;
  var TextureButton2: Texture;
  
  
  function Start() {
      timeRemaining = startTime;
  }
  
  function Awake()
  {
       TextureButton1 = miniButtonTexture;
       TextureButton2 = miniButtonTexture;
      //startTime = 20.0;
      clockBGMaxWidth = clockFG.width * 1;
  }
  
  function Update () 
  {
      if(!clockIsPaused)
      {
         // make sure the timer is not paused
         DOCountdown();
      }
  }
  
  function OnGUI()
  {
      
      var newBarWidth : float = (percent/105) * clockBGMaxWidth; // this is the width that the foreground bar should be
       
          GUI.BeginGroup (new Rect(0, 120, newBarWidth, clockBG.height));
         
         GUI.DrawTexture (Rect (118, 0, clockBG.width * 1, clockBG.height), clockBG);
         GUI.EndGroup ();
         
         
              GUI.BeginGroup (new Rect(0, 0, clockFG.width * 1, clockFG.height));
              GUI.DrawTexture (Rect (0, 0, clockFG.width * 1, clockFG.height), clockFG);
              GUI.EndGroup();
                  
                  if (GUI.Button(Rect(735,20,50,50), buttonTexture, "")){
                 Application.LoadLevel("ThreeD_Overview");
                 
                  }
                 if (GUI.Button(Rect(345,427,64,32), TextureButton1, "")){
                 //Pause Timer
                 clockIsPaused = !clockIsPaused;
                 audio.PlayOneShot(beep1);
                   TextureButton1 = miniButtonTexture2;   //Swap to Texture2
                   
                  }
                  
                  if (GUI.Button(Rect(420,427,64,32), TextureButton2, "")){
                 //Reset Timer
                 audio.PlayOneShot(beep2);
                 TextureButton2 = miniButtonTexture2;   //Swap to Texture2
                 
                 timeRemaining = startTime;
                 
                  }
  }
  
  function DOCountdown()
  {
      //timeRemaining = startTime -Time.time;
      timeRemaining -= Time.deltaTime;
  
      percent = timeRemaining/startTime * 100;
      if (timeRemaining < 0)
      {
         timeRemaining = 0;
         clockIsPaused = true;
         //TimeIsUp();
         Debug.Log("Time is up!");
         Application.LoadLevel ("KnightSpinningLogo_NoKITTintro");
         
      }
      //ShowTime();
  }