Fact- the only way this will work if if you are coloring hair that is "virgin" or has never been colored. And if and only if you are a mix of salt and pepper so that the color will grab differently on the light grey and the contrasting shades of your natural salt and pepper.
If you have previously colored hair and plan on using this product thinking that you will get multi-dimensional hair color it will not work. The only way to get dimensional color is to alternate light and dark colors for example: highlights and low lights. There are some talented individuals out there that can accomplish this task at home or have a friend do it but I highly recommend getting it done by a professional.
I used at home hair color for years before I knew the difference and why it really isn't usually the way to go. Ever picked out that box and really liked the color, then when you get it home it turns out fine but looks nothing like the color on the box? Me too! I could explain why but that would be lengthy and it's harder to understand if you are not used to professional hair color formulation.
Do some research, ask a professional. You can ask me on here if you like. Most Stylists in salons will do complementary consultations. And if they don't find someone that will!