Most Helpful Guy
You might have to dye it a darker colour. If anyone asks, explain it's for Halloween (and then go as something like a witch that needs darker hair). I once had a permanent dye job screwed up and doing it jet black was the only solution.2
Most Helpful Girl
If it's only semi permanent it will wash out. Wash your hair, A LOT, and use clarifying shampoo.
There is also a product called OOPS, it will strip the dye from your hair (it doesn't bleach)3