Hair Colour 101: The Chemistry of Hair Dye and How Hair Colour Works

What actually happens to your hair when you colour it? While you’re sitting in the salon with your hair wrapped in foils, you’re probably thinking more about the finished shade than the chemistry of hair dye happening underneath them. 

But permanent hair colour isn’t simply painted onto the surface of your strands. It relies on a carefully controlled series of chemical reactions that allow colourists to lighten your natural pigment, deposit new colour, and create the shade you see in the mirror.

Here’s what happens inside the hair during colour processing—and why protecting the condition of colour-treated hair matters long after you leave the salon.

Hair is primarily made from a tough structural protein called keratin. Every strand features multiple layers: the cortex, which provides strength and natural colour pigment, and the cuticle, your hair's protective outer shield.

Permanent hair colour needs access to that inner cortex. And that's where the chemistry begins.

Open → oxidise → develop colour.

Think of the hair cuticle as a protective outer shield made from overlapping scales.

In healthy hair, those scales lie relatively flat, helping protect the inner structure of the strand and creating a smoother surface that reflects light. For permanent colour to work, however, the colouring system needs to get beyond that protective barrier.

Alkaline permanent colour causes the hair to swell and the cuticle to become more permeable so the active ingredients can penetrate into the cortex.

This is essential for creating permanent colour, but repeated chemical processing can also leave the cuticle more vulnerable to damage.

That's one reason frequently coloured or lightened hair can become more porous, rough, dry, or prone to breakage over time.

According to celebrity stylist, Bradley Leake:

“Most permanent hair dyes contain ammonia, which elevates the pH of the hair, causing the hair to swell and push the cuticle open.”

Hair dye oxidation is one of the most important—and most misunderstood—parts of permanent hair colouring.

Hydrogen peroxide acts as an oxidising agent during the process. It can oxidise natural melanin, helping lighten the existing pigment, while simultaneously triggering reactions between oxidative dye precursors.

Initially, these dye precursors are small enough to penetrate into the hair. Once inside, they react and combine to create larger coloured molecules.

Those newly formed molecules are much harder to simply wash straight out of the strand, which helps explain why oxidative permanent hair colour lasts significantly longer than temporary colour deposited primarily on the surface.

So when people talk about hair colour oxidation, they’re describing a chemical reaction that plays a major role in both lightening existing pigment and developing new permanent colour.

No, and this distinction is important.

Hydrogen peroxide is an oxidising agent and developer used in many permanent hair colouring and lightening systems. Hair lightener, commonly called bleach, is a different formulation.

Lightening products typically combine peroxide developer with ingredients such as persulfate salts and alkalising agents to produce much more extensive oxidation of the hair's natural melanin.

That stronger pigment-removal process is what allows a colourist to take hair considerably lighter. This also helps explain why dramatic lightening services can have a greater effect on the condition of the hair than simply depositing or subtly shifting colour.

Repeated processing can contribute to:

• Increased porosity and cuticle damage
• Dryness and roughness
• Reduced elasticity
• Greater susceptibility to breakage
• Frizz and a less reflective, less glossy surface

Generally, the more intensive the chemical process (particularly repeated bleaching or significant lightening) the more important your post-colour hair routine becomes.

Your colourist creates the chemistry in the salon; what you do afterwards determines how you care for the resulting hair.

Use formulas designed with colour-treated hair in mind, avoid unnecessarily harsh cleansing, condition consistently and adjust your treatment routine according to what your hair actually needs—whether that's hydration, strengthening or surface repair.

And remember: healthy-looking colour isn't only about preventing fade.

A smoother, better-conditioned cuticle reflects light more evenly, helping hair look glossier and your colour appear more vibrant. Rough, damaged strands scatter light instead, which can make even freshly coloured hair look dull.

That’s the useful takeaway from understanding the chemistry of hair colour: the science that changes your shade can also change your hair—so caring for both colour and condition is key.

Shop All Colour-Extending Products

Keep your colour looking vibrant, glossy and fresh with Color Wow's range of colour-extending haircare products.