Screen.width & Screen.height Becoming button

I found this code in the Unity document and I wanted to use it for my game here is the code:

 /* Screen.width & Screen.height example */
 var isQuitButton = false;
         function OnMouseEnter() 
         {
         //change the coler of the text
         renderer.material.color = Color.green;
         }
         function OnMouseExit() 
         {
         //change the coler of the text
         renderer.material.color = Color.white;
         }
 // JavaScript
 function OnGUI () {
     GUI.Box (Rect (0,0,100,50), "Reset");
 
     GUI.Box (Rect (Screen.width - 100,0,100,50), "Top-right");
     GUI.Box (Rect (0,Screen.height - 50,100,50), "Bottom-left");
     GUI.Box (Rect (Screen.width - 100,Screen.height - 50,100,50), "Bottom-right");
 }

now what i want to do is to place an "OnMouseEnter() " function so that on line 15 the text can change color when the mouse overs and for the text "Reset" to be selectable as well,

This is the code that I'm using along with the first one but I'm getting all sorts of error

 var isQuitButton = false;
 
 function OnMouseEnter() 
 {
     //change the coler of the text
     renderer.material.color = Color.green;
 }
 function OnMouseExit() 
 {
     //change the coler of the text
     renderer.material.color = Color.white;
 }
 
 function OnMouseUp()
 {
     //is it a quit button?
     if( isQuitButton )
     {
         //quit the game
         Application.Quit();
     }
     else
     {
         //load level
         Application.LoadLevel(1);
     }
 }

can somebody help me?