I would recommend
against using bleach until you have first used color remover. Some pigments don't react very well with bleach. Color removers like ColorFix, Color Zap, Color Oops, Color Charm Color Corrector, and Igora Phantom will dissolve the artificial pigment. You can then bleach the hair or recolor it, whatever suits your needs.
http://www.robertcraig.com/php/index...ation=colorfix
There are two types of color remover. One is sulfur-based (chemically, it's a reducing agent). That will dissolve the color. You sometimes have to use it two or three times in order to get all of the pigment out. The second type is bleach-based (chemically, an oxidizer). That type is
extremely hard on your hair and I wouldn't recommend it. If you see something that says "persulfate" or "peroxide" on the ingredient list, don't use it. If you see something that says "...thio..." then it's a sulfur-based product.