What does bleach do?

Bleach does 2 things: it opens your hair cuticles so that the dye will go inside your hair instead of around your hair strand, and it lifts your natural or existing hair colour so vivid shades like pink, blue, and purple can come out brighter and more intense.

Naturally dark hair will bleach to an orange tint due to the large amount of red pigment in the hair. A higher bleach volume is more powerful, has more lift, but can be more damaging.