Lightening Dye Darkened Hair?

I dyed my dark blonde/light brown hair to a medium dark brown about a month ago with a packet semi permanent dye thats supposed to wash out in 20-30 washes (I think... I can't really remember :S) and now I'm over it. Any good ways to lighten it to almost my natural colour without going to a salon? Also, what would happen to my hair if I was to put John Frieda Go Blonder Lightening products into my hair, hopefully make it lighter.. I work at a swimming school so would my hair go green?