l would STRONGLY advice you strip it or try what Jason reccomended. I will let you heed my advice.
I am a natural blonde. I went DARK amber. Then I wanted to go back to blonde. SOOOOOOOO like a moron I bought the color and slapped it on. I now had ORANGE hair. SO I bought a brown color to try to even it out. I was now BLUE/GRAY. I finally stood in the aisle for about 30 minutes reading each box and someone had told me to pick an ASH color that darker skin women use (asians, etc) to go lighter. I finally found one I liked and tried it and I was a nice brown color...GOODLUCK...You might just want to go have it professionally done...If they mess it up they'll fix it =)