You've taken the plunge and gone with a permanent color or a strong semipermanent, but it's just not what you pictured. If you had it done at a salon, any good professional will be happy to adjust it if you're not completely satisfied. (Of course, you might want to think twice about returning to a person who botched the job in the first place.)
If you just can't live with it at all, a colorist can use a color remover to lighten your hair, but you run the risk of further damage and a brassy look and you'll probably have to dye it yet again. (There are color removers available in drugstores, but this process is best left to the pros.) If you've used a gentle semipermanent color, most of it will eventually wash out -- just keep shampooing. In any event, time and a haircut may be your best bets.