How to return to natural hair color?

Six days ago, I went to the hairdresser to dye my bangs navy blue. They applied bleach, applied the dye, and trimmed my hair. There were no problems after leaving the salon, but after six days, my hair looks terrible. The top of my bangs is light blue and the ends are gray. I've decided I'd like to try my natural hair color again because I don't like navy blue anymore. Can I just dye it brown? If not, why should I do?