Vertex Alpha value doesn't update?

I'm trying to show a fading splash screen, So I setted up a strecthed cube in front of the camera(as explained in the guide). Now I want to fade out that cube changin vertices alpha values. The code for this(script attached to the cube):

 void Update () {
         
         float alpha = Mathf.Sin (Time.time);
             
         Mesh myMesh = GetComponent<MeshFilter>().mesh;
         if(myMesh){
             Debug.Log("transparency changed!");
         }else{
             Debug.Log("cube not found!");
         }
             
         Color[] meshColor = myMesh.colors;
         int i;
         
         for(i=0;i<meshColor.Length; i++){
             meshColor[i].a = alpha;
         }
     }

seem's that vertex color is not used at all by any of the material ( I tried all materials and no one showed vertex color): That's a big feature lack! fix that! In FREE version of unity there's no need at all for use "COLORS" component in vertices since no material is using vertex colors => memory waste. Alpha value in vertices will allow for a lot of nice effects even on older fixed-pipeline hardware. Seems a big pity Unity "skipped" that important detail in FREE version.