You could do one of two things, a soap cap (this is a few tbsp of powdered bleach and developer mixed in with shampoo) apply to wet hair and let sit for about 10 minutes. The second option is a bit easier on your hair but not as likely to work. You can make a baking soda shampoo. Baking Soda opens the cuticle to let the color out a bit. Mix a few TBSP of baking soda with shampoo and scrub into wet hair and let sit for like 10 minutes.
Or you can just wait it out. Another option might be to use the dark color, go buy a second bottle, on the back of your hair to blend the two colors better. It won't give you the lighter front you wanted, but it will even things out a bit probably.
Me personally, I'd go have a nice time letting someone else deal with it at the beauty shop, they have the equipment and expertise to help you get the look you want :)