The toner's principle of work is based on enhancing the hair color, giving it a more “toned” look. It works best on pre-lightened hair. Luckily, it is when your gray hair is beneficial: it's already lightened, and pigments can easily penetrate your strands. You can play with the ways of using a toner to cover up your white hair such as completely replacing the color, blending in the shades to match your natural color, etc.