If you really don't like it, you can buy a hair dye remover from a chemist. It is for the semi permenant dyes, and when I used it a few years ago, it def took the bulk of the dye out. Not sure if you could then re-dye or not, you would have to check.
If it's a wash in dye that lasts 6 - 8 washes, then I would think just washing your hair lots would do it.