Hair Is Dead Protein With Living Consequences
Only the follicle beneath the scalp is alive. Everything visible above the skin is composed of dead keratinised cells, which is why cutting hair causes no sensation and why damage to the shaft cannot be repaired biologically.
This has a critical implication for colouring. Any change made to the visible hair is permanent until that section grows out or is cut off, because the fibre has no capacity to heal, regenerate or reverse chemical alteration.
The Shaft Has Three Distinct Layers
A hair fibre is built in concentric layers. The outer cuticle consists of flat overlapping scales resembling roof tiles, forming a protective barrier that determines shine and smoothness.
Beneath it lies the cortex, which makes up most of the fibre's bulk and contains the pigment granules and the keratin structures responsible for strength. At the centre of thicker hairs is a hollow core called the medulla, which has little relevance to colouring.
Natural Colour Comes From Two Melanins
Hair colour is produced by melanin granules embedded in the cortex, made by melanocytes in the follicle and packaged into the growing fibre.
Eumelanin produces brown and black tones, while pheomelanin produces red and yellow. Every natural hair colour is a combination of these two in differing proportions and densities. Blonde hair has little eumelanin, red hair is dominated by pheomelanin, and black hair is densely packed with eumelanin.
Grey Hair Is Hair Without New Pigment
Greying occurs when melanocytes in the follicle become exhausted or die. The follicle continues producing keratin normally, but no pigment is added to the growing fibre.
The result is hair that appears white, since it scatters light rather than absorbing specific wavelengths. What people perceive as grey is usually a mixture of white and still-pigmented hairs. Grey hair also has a different surface texture and resists dye more stubbornly.
Temporary Colour Sits on the Surface
The simplest colour products deposit large pigment molecules on the outside of the cuticle without entering the fibre. Colour-depositing shampoos and hair mascaras work this way.
Because nothing penetrates the hair, these wash out within one or two shampoos. They cannot lighten hair at all, since the natural pigment underneath remains completely untouched, and they show poorly on dark hair.
Semi-Permanent Dye Slips Under the Cuticle
Semi-permanent colours use smaller pigment molecules that pass between the cuticle scales and lodge in the outer cortex. No chemical reaction and no lifting of natural pigment occurs.
These typically last six to twelve washes, fading gradually as the molecules wash free. They contain no ammonia or peroxide, so they cause minimal damage, but they can only add tone on top of existing colour rather than lightening it.
Demi-Permanent Dye Uses a Gentle Developer
Demi-permanent products occupy a middle position. They include a low-volume peroxide developer that allows small dye precursors to enter and react inside the cortex, forming larger molecules.
Because the peroxide concentration is low and no ammonia is used, natural pigment is barely lifted. The result lasts around twenty to twenty-eight washes and is favoured for blending grey, adding richness or correcting tone without a hard regrowth line.
Permanent Dye Requires Two Separate Chemicals
Permanent colour is always supplied as two components that must be mixed immediately before use. One contains dye precursors and an alkaline agent, the other contains hydrogen peroxide.
Mixing begins a chemical reaction with a limited working time, which is why the mixture cannot be stored. Both components are essential: the alkaline agent opens the hair and activates the peroxide, and the peroxide both destroys existing pigment and drives the formation of new colour.
Ammonia Opens the Cuticle
The alkaline agent, traditionally ammonia, raises the pH of the mixture to around nine or ten. Hair is naturally slightly acidic, and this shift causes the fibre to swell and the cuticle scales to lift away from the shaft.
This creates channels through which small molecules can reach the cortex. Without this step, dye precursors and peroxide would remain on the surface. Ammonia-free products substitute alternative alkalis such as ethanolamine, which are less pungent but also less volatile and may linger in the hair.
Peroxide Destroys Natural Melanin
Hydrogen peroxide is a powerful oxidising agent. Once inside the cortex it attacks melanin granules, breaking the pigment molecules apart into colourless fragments.
This is genuine destruction rather than covering. Natural pigment cannot be restored once oxidised, which is why permanent colour creates a visible line of demarcation as untouched hair grows in, and why going darker after lightening requires depositing new pigment rather than reversing the process.
Lifting Reveals Underlying Warm Tones
Melanins do not break down evenly. Eumelanin, which provides cool dark tones, oxidises more readily than pheomelanin, which provides warmth.
As hair lightens, the surviving red and yellow pigment becomes proportionally dominant, which is why bleached hair passes through red, orange and finally yellow stages. This underlying warmth is the reason toners and violet or blue-based products exist, since they neutralise unwanted brassiness.
Dye Precursors Are Small and Colourless
The colour-forming ingredients in permanent dye are not pigments when applied. They are small aromatic molecules, most commonly derivatives of para-phenylenediamine, which are essentially colourless in the tube.
Their small size is precisely what allows them to penetrate the swollen cortex. A finished pigment molecule would be far too large to enter the hair, which is the central problem that oxidative dye chemistry solves.
Oxidation Builds Pigment Inside the Hair
Once the precursors are inside the cortex, peroxide oxidises them so they become reactive and begin joining with coupling molecules also present in the formula.
These reactions build progressively larger coloured molecules through a process of polymerisation. The finished pigments are too large to escape through the cuticle, which is precisely why the colour is permanent. The dye is manufactured in place, inside a structure it could never have entered fully formed.
Different Couplers Produce Different Shades
The final colour depends on which precursors and couplers are combined. Altering the chemical structure of the coupling agents changes the resulting molecule and therefore the wavelengths of light it absorbs.
This is how a single chemical principle produces an enormous palette. Manufacturers blend multiple precursor and coupler combinations in carefully controlled ratios to achieve reproducible shades across many hair types.
Developer Strength Controls Lift
Peroxide is sold in several strengths, commonly expressed in volumes. Ten volume deposits colour with minimal lightening, twenty lifts roughly one to two levels, thirty lifts two to three, and forty is used for maximum lift.
Higher volumes cause proportionally more damage, because the same oxidising power that destroys melanin also attacks the keratin structure and the cuticle. Professional practice favours the lowest developer strength that achieves the required result.
Dye Cannot Lighten Existing Dye
One of the most consistently misunderstood points is that permanent colour will not lift artificial pigment. It lifts natural melanin only.
Someone with previously dyed dark hair who applies a lighter permanent colour will see change only at the virgin regrowth, leaving the previously coloured lengths unchanged. Removing artificial pigment requires bleach or a dedicated colour remover, which is why correction work is difficult and damaging.
Bleach Works Differently From Dye
Bleach uses persulphate salts mixed with peroxide to produce a far more aggressive oxidising system capable of destroying melanin almost completely without depositing any new colour.
It creates the blank canvas required for pastel and vivid shades, but it damages hair substantially. Cuticle scales are eroded, the protein structure is weakened and cysteine bonds are oxidised, which is why over-bleached hair becomes porous, elastic when wet and prone to snapping.
Bond Builders Address a Specific Type of Damage
Bond-building treatments target disulphide bonds, the strong links between sulphur atoms in keratin that give hair much of its structural integrity. Oxidation during bleaching breaks many of these.
These products contain molecules that link broken sulphur groups back together, restoring some structural strength during or after chemical processing. They do not repair cuticle erosion or reverse all damage, but they meaningfully reduce breakage during aggressive lightening.
Grey Hair Resists Colour for Structural Reasons
Grey and white hair frequently refuses to take colour evenly. The cuticle is often tighter and more tightly packed, making penetration slower and less uniform.
Grey hair also tends to be coarser and more resistant, and because it contains no melanin there is no underlying warmth to support the deposited tone, which can look flat or ashy. Colourists address this with pre-softening, longer processing or filling with a warm base first.
Porosity Determines How Colour Behaves
Porosity describes how readily hair absorbs and releases moisture and chemicals, and it is governed by cuticle condition. Healthy, smooth cuticle produces low porosity and resists penetration.
Damaged or chemically processed hair has lifted or eroded cuticle and absorbs colour rapidly, but also loses it quickly. This is why previously bleached ends often grab colour darker than the roots and then fade fastest, requiring adjusted formulation along the length.
Fading Happens Through Several Routes
Even permanent colour fades, because the cuticle is never fully resealed and pigment molecules can gradually escape, particularly during washing in hot water with harsh surfactants.
Ultraviolet light also degrades artificial pigment chemically, as does chlorine in swimming pools. Red shades fade fastest because red pigment molecules are among the largest and are most prone to washing out through an imperfectly sealed cuticle.
Chlorine and Minerals Alter Colour Unpredictably
Swimming pool water can turn light blonde hair green. Chlorine is not the direct cause; the culprit is copper, present from algaecides and from corroding pipework.
Oxidised copper binds to the hair protein and produces a green tint, which is far more visible on porous, pale hair. Chelating shampoos containing agents that bind metal ions remove these deposits, which is also why well water with high mineral content causes persistent colour distortion.
Henna Is Chemically Different and Restricts Later Work
Henna colours hair through lawsone, a molecule that binds directly to keratin without oxidation or lifting. It coats and stains rather than penetrating and reacting.
This makes it gentle, but it also creates a barrier that prevents subsequent chemical colour from penetrating evenly. Compound hennas containing metallic salts are more problematic still, since they can react violently with peroxide, generating heat and sometimes damaging hair severely.
Allergic Reactions Are a Genuine Risk
Para-phenylenediamine and related compounds are recognised contact allergens. Sensitisation can develop at any point, even after years of uneventful use, and reactions range from scalp irritation to severe facial swelling.
This is why patch testing forty-eight hours before every application is standard professional advice rather than a formality. Black henna temporary tattoos, which contain very high concentrations of the same compound, are a documented cause of sudden severe sensitisation.
Colour Fades Toward Its Underlying Base
When permanent colour fades, hair rarely returns to its original shade. The natural melanin was destroyed during processing and cannot come back, so the hair reveals its lightened underlying tone.
This is why faded brown dye often looks orange or brassy rather than simply lighter brown. The artificial pigment has washed away, exposing the warm underlying colour that the dye had been masking all along.
Level and Tone Are Separate Measurements
Professional colour uses a level scale from one, representing black, to ten, representing the lightest blonde. Level describes only depth, meaning how light or dark the hair is.
Tone describes the colour direction at that level, such as ash, golden, copper or violet. Understanding that these are independent explains why two products labelled with the same number can produce visibly different results, and why tone correction uses complementary colours.
Scalp Heat Speeds Processing Unevenly
Colour develops faster where the scalp warms it, which is why roots frequently process more quickly than mid-lengths and ends. This produces the hot roots effect, where regrowth appears noticeably lighter or warmer.
Professional application accounts for this by applying to mid-lengths first and roots last, or by using slightly different formulations along the strand. It is one of the main reasons home application produces uneven results.
Damage Is Cumulative and Invisible at First
Each chemical service degrades the fibre further, and because hair cannot heal, the effects accumulate along the length. Hair at the ends may have undergone many more processes than hair near the scalp.
Early damage is not obvious, appearing first as increased porosity, faster fading and reduced shine. Structural failure follows later, which is why hair can seem to tolerate repeated colouring for a long period and then deteriorate rapidly.
Acidic Aftercare Helps Reseal the Cuticle
Because permanent colouring leaves the cuticle raised by alkalinity, restoring a slightly acidic environment helps the scales lie flatter, improving shine and slowing pigment loss.
This is the function of acidic conditioners and post-colour treatments, and the reason colourists apply them immediately after rinsing. Washing with cooler water and sulphate-free shampoo works through the same principle of keeping the cuticle closed.
Hair Dye Is Ancient
Colouring hair long predates modern chemistry. Henna was used in Egypt and across the region for thousands of years, and Romans used lead compounds and fermented preparations to darken greying hair.
The modern industry began in 1907 when the French chemist Eugène Schueller formulated a synthetic dye based on para-phenylenediamine, founding the company that became L'Oréal. The basic oxidative chemistry he established remains in use today.
Regulations Restrict Specific Compounds
Hair dye ingredients are among the more closely regulated categories in cosmetics. The European Union has prohibited or restricted a substantial list of substances following safety review.
Lead acetate, once common in progressive darkening products, has been banned in several jurisdictions. Concentration limits and mandatory warning labels apply to oxidative dye precursors, reflecting both allergy risk and long-running research into other health effects.
The Chemistry Explains Every Practical Rule
Once the mechanism is clear, the standard advice becomes obvious rather than arbitrary. Dye cannot lift dye because peroxide oxidises melanin, not artificial pigment.
Lightened hair looks brassy because warm pheomelanin survives oxidation better than cool eumelanin. Red fades fastest because its molecules are largest. Colour never fully returns to natural because the original pigment was destroyed rather than covered. Every rule follows from what the chemicals actually do inside the fibre.
Toners Neutralise Rather Than Cover
Toning works on the colour wheel principle that opposite hues cancel one another. Violet neutralises yellow, blue neutralises orange, and green neutralises red, which is why purple shampoos exist for blonde hair.
A toner deposits a small amount of the opposing pigment so that unwanted warmth is visually cancelled rather than hidden under a heavier colour. Because the deposit is light and sits in the outer cortex, toner fades faster than permanent colour and needs regular refreshing.
Balayage and Foils Control Where Lightener Acts
Highlighting techniques exist because applying lightener to the entire head produces a flat, uniform result and maximum damage. Selective application creates dimension while leaving much of the hair untouched.
Foils isolate sections and trap heat, producing faster and more uniform lift from root to tip. Balayage paints lightener freehand onto the surface, leaving the underside darker and creating a softer, grown-out effect that requires less frequent maintenance.
Colour Correction Is a Multi-Step Process
Fixing an unwanted result is rarely a single application. A colourist must first assess what pigment is present, whether artificial or natural, and how porous and damaged the hair already is.
Removing artificial pigment, filling the hair with a missing underlying tone and only then depositing the target shade may take several sessions spread over weeks. Attempting all of it at once frequently destroys the hair, which is why correction costs far more than a straightforward colour service.
Sources
- Wikipedia: Hair coloring β Dye categories, oxidative chemistry, melanin types and developer strengths.
- Britannica: Hair dye β Encyclopedia overview of hair colouring history and chemistry.
- US FDA: Hair Dyes β Official guidance on hair dye safety, allergic reactions and patch testing.
FAQ
Does hair dye coat the hair or go inside it?
Permanent dye goes inside. An alkaline agent swells the shaft and lifts the cuticle so small colourless precursors can enter the cortex, where they react to form pigment.
What does the peroxide actually do?
Two jobs. It oxidises and destroys your natural melanin, and it activates the dye precursors so they join together into larger coloured molecules inside the hair.
Why is permanent colour permanent?
The pigment is built inside the cortex from small precursors. The finished molecules are too large to escape through the cuticle, so they cannot simply wash out.
Why does bleached hair go orange or yellow?
Melanins break down unevenly. Cool eumelanin oxidises more readily than warm pheomelanin, so surviving red and yellow pigment becomes proportionally dominant as hair lightens.
Why can't I lighten previously dyed hair with dye?
Peroxide lifts natural melanin only, not artificial pigment. You would see change at the virgin regrowth while the coloured lengths stay the same. Bleach or a colour remover is needed.
What is the difference between semi and demi-permanent?
Semi-permanent deposits small pigment molecules with no developer and lasts six to twelve washes. Demi uses a low-volume peroxide so precursors react inside the cortex, lasting twenty to twenty-eight washes.
What do developer volumes mean?
They indicate peroxide strength. Ten deposits with minimal lift, twenty lifts one to two levels, thirty lifts two to three, forty gives maximum lift and maximum damage.
Why is grey hair so hard to colour?
The cuticle is often tighter and more resistant, and with no melanin there is no underlying warmth to support the deposited tone, so results can look flat or ashy.
Why does red dye fade fastest?
Red pigment molecules are among the largest and escape most readily through a cuticle that is never perfectly resealed after processing.
Why doesn't faded dye return to my natural colour?
Because the natural melanin was chemically destroyed, not covered. As artificial pigment washes out, the lightened warm underlying tone is revealed instead.
Why does chlorine turn blonde hair green?
Chlorine is not the direct cause. Copper from algaecides and corroding pipes oxidises and binds to hair protein, producing a green tint most visible on porous pale hair.
Why can't you dye over henna?
Henna binds directly to keratin and forms a barrier that prevents even penetration. Compound hennas with metallic salts can react violently with peroxide, generating heat and severe damage.
Is patch testing before dyeing really necessary?
Yes. Para-phenylenediamine is a recognised allergen and sensitisation can develop suddenly after years of safe use. Reactions range from irritation to severe facial swelling.
What are hot roots?
Scalp heat speeds development, so roots process faster and appear lighter or warmer than the lengths. Professionals apply to mid-lengths first and roots last to avoid it.
What do bond builders actually do?
They reconnect disulphide bonds between sulphur atoms in keratin that oxidation breaks during bleaching. They reduce breakage but do not repair cuticle erosion or reverse all damage.
About the Author
We reference Wikipedia and other authoritative sources to explain the background and current understanding of this topic.
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