Don't panic: During the first week after coloring, hair usually lightens a bit, especially after a few shampoos; this is particularly true of coarse and damaged hair. If you've used permanent color and you just can't live with it, a colorist can use a color remover to lighten your hair. (There are color removers available in drugstores, but this process is best left to the pros.) However, this should be your last resort -- you'll risk further damage and a brassy look, and you'll probably have to dye it yet again. If you've used a gentle semipermanent, just keep shampooing; eventually, most of it will come out.
To keep your hair from getting too dark when coloring at home, choose a product that's a shade or two lighter than the one you think you want, especially if you have coarse, damaged or thin hair -- these all absorb more dye than other types. A deep conditioning before you color also can help. Color builds up, too -- even semipermanents -- so when you do touch-ups, don't let the product stay on the ends of your hair too long. And always do a strand test to get the timing right.