Arlene,
a couple of years ago at work I had to research a similiar situation. The consensus at that time seemed to be that it was almost impossible to change a dark grout color to a much lighter color so we did not go forward with the project. It's possible that there have been new products introduced now that will accomplish this. At the minimum, I would ask the company for a list of clients in your area who have had this work done and contact them to see if they are satisfied. Its very easy to make pictures on a web site look good. Good luck.