Large colour molecules on the outside of the cuticle. Lasts one shampoo. Can stay longer if the hair is porous.
Large and small colour granules penetrate the cuticle and lie on the outer edge of the cortex. Lasts 6 shampoos. Can only darken or tone.
The addition of weak hydrogen peroxide allows colour to enter the outer cortex. Lasts up to 24 shampoos and gradually fades (mixed molecules).
Tiny colour granules are mixed with hydrogen peroxide. The small colour granules swell, join together and become trapped deep in the cortex. Grows out.
Natural colour molecules (melanin) are oxidised to form colourless oxy-melanin. Grows out.