ahh, a department i have much experience in!
You're stuck. You need to buy a bottle of peroxide and bleach your hair. you'll only need to bleach it for a few minutes, though since your hair is light. Go to a salon if you are afraid of screwing this up. It's gonna cost you, but it's better than screwing up and frying your hair, getting a chemical burn, or worse.
however, if you've got qualified assistance (someone who can help you that you trust enough ot not screw up), you can buy a pouch of lightening powder and a bottle of creme developer from any beauty supply store. if you're not sure what products to get, ask the shop keeper. But my suggestion would be to get a creme no higher than 30 volume (20 should be good enough) and a strong powder for faster action.
Mix the two in a non-metal bowl or cup till it's about the thickness of heavy cream. Brush into your hair with a dyeing brush and let sit till it's white.
After that, you can buy regular hair dye and dye your hair back to normal.
Color is for crayons, not for people.