How to change GUI.DrawTexture with OnMouseEnter function?

So I'm self teaching myself, and this is the first kink along the way, and I was wondering what is the best way for me change the texture that made with GUI.DrawTexture by using OnMouseEnter and OnMouseExit so that I can change the texture to something else when the mouse is hovered over it, and then reset when it exits?

Normally, if I was doing this using the scene itself, I'd have no problem with it, but for something done entirely in a script, I'm really confused.

 // Allows you to disable to the gui when you want by setting this value to false.
 var guiDraw : boolean = true;
 // Allows you to set what texture you want the background to be.
 var controlBackgroundTexture : Texture2D;
 // Allows you to set what texture you want the start button to be when it is not entered in by the mouse.
 var controlStartNotEnteredTexture : Texture2D;
 // Allows you to set what texture you want the start button to be when it IS entered in by the mouse.
 var controlStartEnteredTexture : Texture2D;
 // Allows you to set what texture you want the exit game to be when it is not entered in by the mouse.
 var controlExitNotEnteredTexture : Texture2D;
 // Allows you to set what texture you want the exit game to be when it IS entered in by the mouse.
 var controlExitEnteredTexture : Texture2D;
 
 // Begins drawing the GUI.
 function OnGUI(){
     // Checks to see if the GUI is enabled, and if it is, it will display it.
     if(guiDraw == true){
         // This draws the background centered and scaled to be on any resolution.
         GUI.DrawTexture(Rect(0, 0, Screen.width, Screen.height), controlBackgroundTexture, ScaleMode.ScaleToFit, false, 0);
         // This draws the start button to be centered on any resolution.
         GUI.DrawTexture(Rect(Screen.width/2.5, Screen.height/2.5, Screen.width/2, Screen.height/2), controlStartNotEnteredTexture, ScaleMode.ScaleToFit, true, 0) && {
             function OnMouseEnter(){
                 guiTexture.Texture = controlStartEnteredTexture;
             }
         }
         // This draws the exit button to be centered on any resolution.
         GUI.DrawTexture(Rect(Screen.width/2.5, Screen.height/1.6, Screen.width/2, Screen.height/2), controlExitNotEnteredTexture, ScaleMode.ScaleToFit, true, 0);
     }
 }

Now, if you tell me that I should just use OnMouseExit before GUI.DrawTexture, I'm seriously going to slam my head in my desk, because I might make this a little more difficult that what I am actually thinking of.