Unfortunately, the reality is quite different. This means colors the non-gray hair just as well as the gray. In the end, by the non-gray hair is darker than before, while the gray hairs to be significantly lighter in comparison. The longer allowed to act the means, the darker the color, no matter what hair color you had chosen. The best results I achieved with "too dark" color and a shorter exposure time, the worst with bright color and long exposure: The colors brown and Ashen about dyeing gray hair a little, while they make the non-gray hair much darker. This difference between Färbwirkung on gray and non-gray hair is smaller, the darker the hair color is selected (tried: Ashy, light brown, medium brown and dark brown). In all cases, you have but dait expected that the treated sections of hair are much darker than before, if you want to reach a wide coloring.
I myself have light brown hair, I can well imagine that these products work better with darker hair, since the darkening of the non-gray hair is expected to fall less important.