Hair colouring could be classified into permanent, semi-permanent and temporary. It is due to the types of hair dye used.
The two latter ones use hair dyes derived from natural plants and minerals. The colour wears off after repeated washing.
Currently, most of the popular hair dyes in the market are permanent hair dyes which uses a combination of several chemicals. They generally contain ammonia, hydrogen peroxide, and p-Phenylenediamine which is a common allergen.
First, the ammonia open up the hair cuticle, making it easier for other ingredients to take effect. The hydrogen peroxide comes in next to bleach the hair. Eventually, the p-Phenylenediamine interacts with the ammonia to form a new colour.
Human’s hair cuticles made of millions and millions tiny scales that are layered on top of one another. It is the protective layer. If the scales are arranged nicely in a correct angle, it will reflect the largest amount of light. That’s why one’s hair looks smooth and shiny.
In the process of colouring your hair using permanent hair dye, some ingredients in the hair dye will open up the cuticle in order to deposit dye. If you use chemical hair dyes frequently, it will damage your hair and cause dry, brittle hair. Your hair will appear dull and lack-lustre which wouldn’t be nice.