Fade in and out a GUIText pressing another GUItext?

In my scene, there are 3 GUIText, when I press the first button of them the other two will fade in, i made something like this this script its in the first button) :

var showtext:boolean = false;
var buttonSP = gameObject;
var buttonMP = gameObject;

function Start(){
 buttonSP = GameObject.Find("ButtonSP");
 buttonMP = GameObject.Find("ButtonMP");
 Fade();
}

function OnMouseDown(){
  showtext = true;
  Fade();
}
 
function Fade(){

if (showtext == false){
while (guiText.color.a > 0){
     buttonSP.guiText.material.color.a -= 10.1 * Time.deltaTime * 20;
     buttonMP.guiText.material.color.a -= 10.1 * Time.deltaTime * 20;
   yield;
    }
  }
if (showtext == true){ 
while (guiText.color.a > 1){
     buttonSP.guiText.material.color.a -= 0.1 * Time.deltaTime * 20;
     buttonMP.guiText.material.color.a -= 0.1 * Time.deltaTime * 20;
   yield;
    }
  }

}

But nothing apears and I can´t find any other way to do this, ¿any idea?.

Use Fade, specifically the Alpha function.