The reason this happened is because the colors mixed, when the regrowth had no color and therefore was able to obtain the proper shade.
In order to give you real advice on how to fix this, a picture is very much necessary. However, I can give you a few quick tips so that an experience like this won't happen again.
1. Read reviews of salons before you go to them.
Yelp is an extremely helpful tool to find the best of practically any store or service. You might specifically look for reviews that mention fantastic color.
2. Go to a salon to get this fixed. Professionals in a well managed salon will know what they are doing, and will fix this problem better than you likely can. Call ahead and ask the receptionist if the salon has a stylist who specializes in corrective color, and make an appointment with this person.
3. If you like coloring your own hair, and the regrowth blends well, let it grow out. This way, if you want a color change, the stylist can simply cut the hair that is not regrowth, and color fresh hair.