Button color in client RPC

I am working for a simple turn multiplayer game.
What I need is that when a player click on a button, this is sent by a server RPC that check the selection and call a client RPC. In this client RPC I need to change the color of that button.
My problem is that the button doesn't change the color.
Here the code:

Code (CSharp):

1.  
2. [ClientRpc]
3.     public void SetActivePlayerClientRPC(ulong id,Matrix m)
4.     {
5.         table = m;
6.         for (int i = 0; i < 4; i++)
7.             for (int j = 0; j < 4; j++)
8.                 if (m.matrix[i, j] != -1)
9.                     MatrixObject.Find(string.Format("{0}-{1}", i, j)).GetComponent<Image>().color = mapColors[ulong.Parse(m.matrix[i, j].ToString())];
10.     }
11.  

The MatrixObject is a gameobject is found this way (in every client is the only tagged "Matrix"):

Code (CSharp):

1. _matrixObject = GameObject.FindGameObjectsWithTag("Matrix")[0].transform;

I tryed to color the button in other points of the code and it works. I can't understand why in this part of the code it doesn't work.

Any ideas?
Thanks

 

Thanks, but same result. What I can't understand is that the MatrixObject is not null and the Find(string.Format("{0}-{1}", i, j)).GetComponent<Image>().color take the value I provided. So I can't figure why the button don't change color.