Sprite alpha can't change during runtime

Hi, my Game Controller class have bunch of codes. I want to change my GameObject alpha property during runtime. Some parts of my code is below.

Brief of my code is here about changing alpha, I assigned these three game object from hierarchy, but not changing alpha.

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 public GameObject firstGO, secondGO, thirdGO;
 
 float fullAlpha = 1.0f;
 float halfAlpha = 0.5f;
 
 public float firstAlpha, secondAlpha, thirdAlpha;
 
 void  Awake (){
 firstAlpha = firstGO.renderer.material.color.a;
 secondAlpha = secondGO.renderer.material.color.a;
 thirdAlpha = thirdGO.renderer.material.color.a;
 }
 
 void Update(){
 
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 updateScore (100);
 
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 }
 
 
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 public int updateScore ( int length  ){
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 firstAlpha = fullAlpha;
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 }