When colouring the hair to the same or to a darker shade new synthetic pigment molecules are added to the hair’s natural pigment changing its colour. Whereas in lightening the hair’s natural colour the darker natural pigments need first to be broken down then replaced with the new synthetic pigments.
Ammonia an alkaline compound with oxygen and hydrogen lifts and opens the hair cuticles allowing the small dye molecule to penetrate the inner cortex. As the dye molecule is oxidised by the hydrogen peroxide the ammonia that provides the alkaline environment acts as a catalytic agent bonding and breaking down the hairs natural colour pigment. The various combinations of pigment colour mixture of the primary colours provide the final shade chosen. Hence when ColourB4 is used to remove the synthetic pigment the hair does not go back to the original pigment as it was removed due the lightening process.
Dye pigment molecules in permanent colour are very small and are designed to easily penetrate deep into the hair’s cortex. But naturally these molecules would simply fall back out of the hair during shampooing. In order to trap the dye molecules in the cortex the dye uses ammonia mixed with hydrogen peroxide.
Whilst the small dye molecule is within the hair cortex an oxidation reaction takes place and begins to swell the dye molecule to a much larger size. Eventually the molecule is so large that it is trapped and unable to escape back through the hair’s cuticle. Think of this as an Alice in Wonderland effect where Alice is unable to back out of the door she came in due to growing so large.