Changing Texture2D at runtime, but it doesent work?

Hey, im just drawing some textures(ability buttons) to the screen, and if an ability is on cooldown then I just want to change the texture of the "button" to my cooldown texture but I cant get it to work.. here is my code:

 private Target _targetScript;

 private Champion_A__FirstAbility _firstAbility;

 private Champion_A__SecondAbility _secondAbility;

 private Champion_A__ThirdAbility _thirdAbility;

 private Champion_A__UltimateAbility _ultimateAbility;


 public float _hpBarLength = 0;

 public Texture2D _hpBarTexture;
 public Texture2D _manaBarTexture;
 public Texture2D _backgroundBarTexture;

 public Texture2D _firstAbilityTexture;
 public Texture2D _secondAbilityTexture;
 public Texture2D _thirdAbilityTexture;
 public Texture2D _ultimateAbilityTexture;

 private Texture2D _firstTexture; 
 private    Texture2D _secondTexture;
 private Texture2D _thirdTexture;
 private Texture2D _ultimateTexture;    

 public Texture2D _abilityCoolDownTexture;

 private float _abilityButtonWidth;
 private float _abilityButtonHeight;

 // Use this for initialization
 void Start () {
     _abilityButtonWidth = _firstAbilityTexture.width * 0.5f;  // They are 2 big, so I felt like making them half the size. hence " * 0.5f "
     _abilityButtonHeight = _firstAbilityTexture.height * 0.5f;

     //Getting the scripts.
     _targetScript = (Target)GetComponent<Target> ();
     _firstAbility = (Champion_A__FirstAbility)GetComponent(typeof(Champion_A__FirstAbility));
     _secondAbility = (Champion_A__SecondAbility)GetComponent<Champion_A__SecondAbility> ();
     _thirdAbility = (Champion_A__ThirdAbility)GetComponent<Champion_A__ThirdAbility> ();
     _ultimateAbility = (Champion_A__UltimateAbility)GetComponent<Champion_A__UltimateAbility> ();

 }
 
 // Update is called once per frame
 void Update () {
     float percentOfHP = _targetScript.Health / _targetScript.MaxHealth;
     _hpBarLength = percentOfHP * 100;
 
     
     CoolDownCheck();
 }

 void OnGUI()
 {
     CoolDownCheck();

     if (_targetScript.Health > 0)
     {
         GUI.DrawTexture(new Rect(100, 10, 0, 10), _backgroundBarTexture);
         GUI.DrawTexture(new Rect(100, 10, _hpBarLength, 10), _hpBarTexture);
     }


     // First Ability
     GUI.DrawTexture(new Rect((Screen.width/2) - 160,
                              Screen.height - 100,
                              _abilityButtonWidth,
                              _abilityButtonHeight),
                     _firstTexture);

     // Second Ability
     GUI.DrawTexture(new Rect((Screen.width/2 - 80),
                              Screen.height - 100,
                              _abilityButtonWidth,
                              _abilityButtonHeight),
                     _secondTexture);

     // Third Ability
     GUI.DrawTexture(new Rect((Screen.width/2),
                              Screen.height - 100,
                              _abilityButtonWidth,
                              _abilityButtonHeight),
                     _thirdTexture);
 
     // ULTIMATE
     GUI.DrawTexture(new Rect((Screen.width/2 + 80),
                              Screen.height - 100,
                              _abilityButtonWidth,
                              _abilityButtonHeight),
                     _ultimateTexture);

 }


 private void CoolDownCheck()
 {
     _firstTexture = _firstAbilityTexture;
     if(_firstAbility.AbilityReady())
     {
         _firstTexture = _abilityCoolDownTexture;
     }
  

  
     _secondTexture = _secondAbilityTexture;
     if(_secondAbility.AbilityReady())
     {
         _secondTexture = _abilityCoolDownTexture;
     }
  

  
     _thirdTexture = _thirdAbilityTexture;
     if(!_thirdAbility.AbilityReady())
     {
         _thirdTexture = _abilityCoolDownTexture;
         Debug.Log("VI KOMMER IN HÄR, VI SKA HA COOLDOWN TEXTURE!!");
     }
  

  
     _ultimateTexture = _ultimateAbilityTexture;
     if(!_ultimateAbility.AbilityReady())
     {
         _ultimateTexture = _abilityCoolDownTexture;
     }
      
 }

the "Target" script is actually where I get my own hp, incase someone would ask about that.

At first I had methods for returning the texture, that I used in the GUI.DrawTexture()... so instead of

GUI.DrawTexture(new Rect((Screen.width/2) - 160, Screen.height - 100, _abilityButtonWidth, _abilityButtonHeight), _firstTexture); <--- this

I had

GUI.DrawTexture(new Rect((Screen.width/2) - 160, Screen.height - 100, _abilityButtonWidth, _abilityButtonHeight), FirstAbilityTexture()); <--- this

private Texture2D FirstAbilityTexture(){

  if(isOnCoolDown)
  {
       return _abilityCoolDownTexture;
  }

  return _firstAbilityTexture;

}

but that didnt work either.

Please help a newbie programmer! :)