Colors were always something she could not understand, no matter how hard he tried to explain them to her. And she wouldn't care about them, except they seemed to such an important part of his world.
He used them to describe everything, from objects to even emotions. She had felt for the first time betrayed by her lack of sight, that she could not experience these descriptions of the world as he saw it.
And it hurt him when he saw her frustration. He could not stand to see her rare moments of self doubt. They were more painful for him then her.
So he stopped his habit. He rarely ever mentioned the color of anything. Instead he switched to using emotions to convey his visual understanding of an object.
They never spoke of this, and it became almost a code between them. One that Toph could use just as easily back.
And for this they were both grateful.