Bleach permanently lightens the color of the hair by physically removing pigment from where it is applied. Since it is not a dye, bleach will not fade from the hair. Bleaching opens the cuticle of the hair which then allows added pigment to stick more easily - just be cautious to avoid over-processing, which can result in permanent damage to your hair. To go back to your natural color, re-dying the hair to a similar shade or growing the hair out are your only options.