If your hair keeps going matte and green then you probably need to change the formula you are using. If your hair is 75-100% white then it might have trouble holding on to warmth. I recommend using gold, copper or red based browns to counteract this green hue you are experiencing.
L’Oreal Professionnel Inoa Supreme would be suitable as it is formulated to cover white hair whilst keeping your colour rich by reflecting light, so it doesn't appear flat and dull.