I once had green hair...I had asked my hair-stylist to get rid of my purple hair I had so I could go for a more neutral color (I was going to a funeral). She rinsed the purple out fine, but the blue dye from MONTHS prior was still there underneath. She bleached it for as long as she could, and it never came out, just turned green. So we had to dye it a darker brunette color (I had wanted blonde) and it still looked green, so she had to layer in some red toner in as well. My hair was disgusting after all that, I had to use a special hair mask for like a month.