What color oops does is remove the chemical bonds of the hair COLOR from your hair strands. Once the color is removed, you are left with fairly ORANGE hair, because every box dye uses ammonia and peroxide to bleach your hair prior to depositing it's color.
So you can effectively remove the coloring from your hair, but if you had.....say, natural brown hair, you won't get that back. You'll have faded, lightened orangish hair. From there, you can then use a dye to achieve a natural color you're looking for.
Hope that makes sense. And you may need 2 bottles. It works very quickly, warms up and you don't let it sit for very long (I think the max is 10 mins).