Hi! I did pretty much the same thing, I've only ever used semi permanent hair dyes.
Naturally, my hairs a mid-blonde color and I dyed it to a chocolate brown color, nothing seems to help get rid of it! Two months later and its still the same color only now I have about an inch of blonde roots coming through, it looks awful!
Any help would be really appreciated!

I've heard that mixing 1 part vitamin C tablets crushed, with 2 parts dandruff shampoo, into a paste, and then left in for an hour, will help the matter. It will not alter the original color underneath.

I've heard good things about ColorZap (http://reviews.sallybeauty.com/6151/SBS-500495/reviews.htm)

Good luck!!