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How Does Hair-Dye Work?

Updated: Dec 16, 2020

Have you ever died your hair? Permanent, or semi-permanent? What is the science behind hair dye, and how does it colour your hair?

The Structure of the hair

To understand how hair dye actually work, we first need to know the structure of a hair shaft.

A hair strand is made up of three main areas:

  1. The cuticle on the outside

  2. The cortex in the middle

  3. ... And the medulla in centre.

The Cuticle

This outermost layer is made up of overlapping tile-like cells. The cuticle protects the hair shaft from losing moisture and flexibility. A healthy cuticle lays flat and remains shiny.

The Cortex

This is the main part of a hair, it contains long strands of keratin held together by strong disulphide bonds as well as melanin - making the hair coloured.

The Medulla

(Only appears in thick hair strands). Made up of transparent cells and air spaces - making it very soft.

How does permanent hair dye work?

  1. Ammonia or highly alkaline solution is used to open up the cuticle.

  2. Peroxide bleaches the hair by turning pigments colourless.

  3. Dye precursors is applied and enters the cortex.

  4. A coupler molecule binds with the precursor, forming a large molecule which cannot exit the cortex.

  5. The hair shaft is conditioned and the cuticle closes.

How does semi-permanent hair dye work?

Semi permanent hair dye is definitely not as complicated as permanent hair dye. The cuticle is not opened at all - instead, a colourant is applied to the shaft which gets in between the scale-like cuticle and stick to the cortex.

The physical structure of the hair was not changed, therefore the dye will be slowly washed out and the hair will go back to normal in 6-10 weeks.

Best dye option for you?

Depending on what you're looking for, here are some pros and cons of each

Permanent hair dye


Will remained coloured until the hair falls out or grow out.

No additional applications are needed.


It will damage the cuticle.

Semi-permanent hair dye


Is less damaging to the hair.



Colour will fade over time.

Re-applications are necessary.

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