Whatever you do! DO NOT turn to putting lemon juice in your hair!! The lemon juice dries it out and since your hair is darker then how it used to be, it WILL turn your hair orange! Trust me i tried it and my hair looked like Ronald Mcdonald on crack -_-
These are the remedies i found for you!
Shampoo your hair with a clarifying shampoo mixed with about a tablespoon of baking soda...it will make a kind of paste...apply it like you always do shampoo, work it in and suds it up...leave on for 10 minutes, rinse. Condition well with a good moisturizing conditioner as it will leave it feeling stiff if you don't. This will not work overnight but will slowly fade the color. Do this a couple times a week.
Try LíOrealís Color Zap for a quick at-home reversal of your color blunder. This easy-to-use kit works after one application, gently removing dye from your hair while leaving your natural hair color intact and ready to re-color. This product works even on permanent hair dye.
Clarifying shampoos not doing it for you? Try using Head and Shoulders dandruff shampoo. This shampoo has long been known for its ability to strip color from hair. If you really need to fade your too-dark hair quickly, try shampooing with Head and Shoulders until your hair fades to the desired shade.
I've actually have had experiences with the 3rd method. I had dyed my tips black (and i mean BUH-LACK). It was permanent so my sister told me it will stay on there for a while, so i was happy. At the time, my mom had bought Head & Shoulders shampoo and conditioner because my sister was having trouble with dandruff. So that means we all had to use the shampoo. I used it and about a 3-6 weeks later my black tips vanished :O I was so mad! But yea, that was my experience with Head & Shoulders ^-^
Good luck with your hair!