I have had my colored at a salon then done it at home and haven't gotten any crazy results.

If you're really worried though, go to a beauty supply shop and buy something to strip your hair. This will take out whatever they put in and you can do with it as you please. But, be ware, your hair will look kind of brassy with the stripping agent so adjust the color for what you hair will be AFTER you strip it. It might also hold the color better so don't go too dark or it might come out darker than you want.