It is the scene that nightmares are made of: you take off the towel to reveal your new hair colour and--gasp! --it's way too dark. The good news is that even if your hair is a completely different shade than you were aiming for, there are things you can do to lighten it up to achieve the colour you were going for in the first place.
Wash your hair with a clarifying shampoo immediately. Clarifying shampoos are made with surfactants, which strip hair colour. The faster you can get this on your hair after it has been dyed, the better the chances are of it stripping away some of the colour.
Call the number on the box of the hair dye. The people that answer the hotline number are trained in these sort of situations and they may have advice for you that helps your specific situation and hair type.
Buy L'Oreal Color Zap or a similar colour-removing product. These are made to take out the hair dye.
Talk to your hairdresser immediately. Tell her that this is not the shade you were going for.
Follow your hairdresser's instructions. She may want to wash and rinse your hair right away to see if some of the colour will come out.
Re-dye your hair, if you and your hairdresser see that nothing else is working. She has professional, good quality dyes, so they should work to cover up that darker shade.