Why Hair Looks Dull

Five causes, and what shine actually is.

Shine is usually described as a sign of healthy hair. It is really an optical effect — a question of how light leaves the surface of each strand — and that difference matters when you are trying to work out why yours has gone flat. Four of the five causes below can be dealt with at home. One cannot, and we would rather tell you which.

Updated 23 September 2026.

Shine is light leaving in one direction

When light meets a strand of hair, some of it bounces straight off the outer surface, the cuticle. That light stays white: it never enters the hair, so the hair’s colour cannot touch it. The rest travels into the strand, is partly absorbed by pigment, reflects off the far side and comes back out tinted the colour of your hair.

Two ways light leaves a hair A strand of hair drawn from root on the left to tip on the right. A ray of light arrives from the upper left. Part of it reflects off the surface and leaves white. Part of it enters the strand, reflects off the far side, and leaves tinted brown at a slightly different angle. root tip light in reflected at the surface: stays white through and back: takes the hair’s colour
Simplified. The two paths leave at slightly different angles because the cuticle’s scales are tilted toward the tip.

Because those scales tilt by around three degrees, the two reflections do not leave at quite the same angle. On glossy hair under a single light you can sometimes see both: a bright white band, and a fainter coloured one sitting slightly further toward the ends.

It is contrast, not brightness

What your eye reads as shine is not simply how much light the hair reflects. It is how strongly that white surface reflection stands out against the softer glow coming back from inside the hair, and how narrow the bright band is. In 1993, researchers at Colgate-Palmolive turned this into a measurement that tracked people’s own judgements of shine closely. Their formula is still used in the industry today.

Shine = S ÷ (D × W) S is the light reflected at the surface, D the light scattered back from inside, and W the width of the bright band.

Three things follow from it, and between them they explain most of what people notice.

  • Smoothness. A flat cuticle sends the surface light off in one direction. A raised or chipped one scatters it, so the band widens and fades.
  • Alignment. Strands lying parallel line their highlights up into one band. Frizz and tangles break it into scattered sparkle.
  • Colour. Dark hair absorbs most of the light that enters it, so little comes back as glow and the white band stands out. Light hair sends much more back, so the same surface looks less glossy on blonde than on black.
Light on a flat cuticle leaves in one direction; light on a raised cuticle scatters. flat cuticle one direction raised cuticle scattered
Smoothness decides whether the surface light leaves together or apart.
The same white highlight painted on a dark brown swatch and a light blonde swatch. It stands out far more on the dark one. dark light the same highlight on both
Identical highlights. The difference you see is contrast alone.
Long, straight dark brown hair seen from behind in soft light, with a clear band of shine across the crown.
Dark
Long, layered light blonde hair seen from behind in similar soft light, where the shine blends into the overall brightness of the hair.
Light
The same effect on real hair, in similar soft light. On the dark hair, the band of shine across the crown stands out clearly. On the light hair, reflections of the same kind blend into the brighter glow coming back from inside it. The two heads also differ in texture, and that affects shine as well.

That last point applies to any hair colour, ours included. Going darker makes hair look glossier even if nothing about its surface has changed.

Under the microscope

The cuticle is a stack of thin, overlapping cells, several layers deep, covering the whole length of the strand. These are our own photographs at 500× magnification.

Hair strand at 500 times magnification with cuticle scales lying flat and close to the shaft.
Cuticle lying flat. The scale edges sit close to the shaft and the surface is continuous, so light leaves it together.
Hair strand at 500 times magnification with some cuticle scales lifted and broken.
Cuticle lifted and broken. Scale edges stand up and the surface is uneven, so light leaving this strand scatters.

Five causes of dull hair

Four of them are about something on or around the hair, and can be dealt with. One is about what has already happened to it.

A film on the surface

Fixable

What’s happening. Conditioner, silicones, oils, styling products and dry shampoo leave a layer on the strand. A thin, even film can add gloss for a while. As it builds up unevenly it does the opposite, because the surface light is now leaving whatever sits on the hair rather than the cuticle. In the 1993 shine study, the worst dulling of all came from particles a shampoo left behind.

What to do. Clarify with a cleanser that leaves nothing behind. Silicones are designed to survive washing — look for ingredients ending in -cone, -conol, -silane or -siloxane — and established build-up can take more than one wash. The same film is the most common reason plant colour does not take, so it is worth clearing before you colour in any case.

Eight reasons natural dyes do not dye →

Minerals from your water

Fixable

What’s happening. Hard water leaves calcium and magnesium on the fibre, where they behave like any other residue. Soap is affected as well: in hard water it forms insoluble calcium and magnesium soaps — soap scum — which can stay in the hair. Dissolved iron does something different. It reacts with plant polyphenols to form dark complexes, which can pull a plant colour duller or greyer than expected.

What to do. A chelating or acidic rinse removes mineral build-up, and filtered water for mixing plant colour takes the iron out of the question. If you wash with a true soap, ours included, follow it with an acidic rinse.

Hard water and plant colour →

A cuticle left raised

Fixable

What’s happening. Hair responds to pH. An alkaline wash swells the fibre and lifts the edges of the cuticle slightly, and that surface feels rougher and scatters more light. Our own Shampoo Bar is a true soap and reads pH 9.35, so this applies to it as much as to anything.

What to do. Finish with an acidic rinse. Our Hair Treatment Powder reads pH 3.2 to 3.5; diluted vinegar or citric acid work on the same principle. The raised state is temporary, and hair feels smoother as the pH comes back down.

Why hair feels rough →

Ends that have worn

Not fixable — only cut

What’s happening. The cuticle wears away gradually along the strand, a process researchers call weathering. Near the root its layers are intact; toward the tips they are thinner, chipped and in places gone, because the ends are the oldest part of the hair. On shoulder-length hair that is roughly two years of washing, brushing, sun and colour. Bleach and perms add a change of their own: they turn the bonds that hold keratin together into cysteic acid, which does not revert.

What to do. Honestly, nothing rebuilds it. Hair is dead tissue with no way to repair itself, and no product, ours included, puts back a cuticle that has worn away. A coating can make worn ends look smoother for a while; a trim removes them. What you can decide is what the new growth goes through.

What peroxide does to the protein →

Heat, sun and friction

Preventable

What’s happening. These are what cause the wear above. Hot tools and close, hot blow-drying damage the cuticle. Ultraviolet light breaks down hair protein and pigment over a summer. Friction from rough towel-drying and heavy brushing chips the scales, and wet hair is the most vulnerable, because a swollen fibre is easier to damage.

What to do. Blot rather than rub, detangle gently with a wide-tooth comb, keep heat lower and further away, and cover your hair in strong sun. None of this restores what has already worn. All of it slows the next inch from wearing the same way.

Shine is not proof of health

Because shine is a surface effect, it can be produced without anything about the hair improving. Silicone serums and oils fill in a rough surface and give it a smooth, reflective skin, so worn hair under a good coating can look glossier than healthy hair without one. And as above, a darker colour raises the contrast that reads as shine, whatever the surface is like.

The reverse is true as well. Healthy blonde, grey or tightly curled hair can look less glossy than damaged dark, straight hair, simply because of how light leaves it. Low shine on its own does not mean something is wrong.

What we have not measured

We have not measured the shine of hair coloured with Radico, and we have not found anyone else who has published that measurement either. Customers often tell us their hair looks glossier afterwards, and we do not doubt them. But on grey or light hair, a darker result raises contrast by itself, so we cannot say how much of that is colour and how much is surface.

What we can say from the formula is narrower. There is no alkali, ammonia or peroxide in a Radico pouch, so nothing in it is there to lift the cuticle or oxidise keratin. Plant colour is not a treatment, and it will not undo worn ends. If we ever run the measurement, the result goes on this page whichever way it falls.

What helps

None of these repairs hair. The first three each do one job against one of the causes above. The last is about the hair you have not grown yet.

Radico Hair Treatment Powder with the herbs it contains.Hair Treatment PowderThree fruits, mildly acidic. The acidic rinse for a raised cuticle, and after any true soap. Radico Shampoo Bar with the herbs used in it.Shampoo BarA true soap that leaves no conditioning film. Alkaline, so pair it with an acidic rinse. Amla fruit with Radico Amla.AmlaAn acidic fruit powder for the lengths, which also cools a henna result.

If you colour your grey

Bleach and peroxide dyes are one source of the wear described in the fourth cause: the change they make to keratin does not revert. If you colour regularly, what you colour with decides what each new half inch goes through.

Radico is plant colour for grey hair. There is no peroxide, ammonia or alkali in it, so it does not make that change. It has limits worth knowing before you buy: it cannot lighten, grey takes it a shade lighter and warmer than the rest of your hair, and results on bleached or permed hair are harder to predict. A darker result will probably look glossier on grey, for the contrast reason explained above, not because anything has been repaired. Patch test 48 hours before every new shade.

See the shades →   What is in organic hair colour, and what it does to hair →

References

Stamm RF, Garcia ML, Fuchs JJ. The optical properties of human hair. I. Fundamental considerations and goniophotometer curves; II. The luster of hair fibers. J Soc Cosmet Chem. 1977;28. The white surface reflection, the coloured internal reflection, and the tilt of the cuticle scales.

Reich C, Robbins CR. Light scattering and shine measurements of human hair: a sensitive probe of the hair surface. J Soc Cosmet Chem. 1993;44:221–234. The shine formula above, and the dulling caused by deposits.

Marschner SR, Jensen HW, Cammarano M, Worley S, Hanrahan P. Light scattering from human hair fibers. ACM Trans Graph. 2003;22(3):780–791. The offset between the white and coloured highlights.

Robbins CR. Chemical and Physical Behavior of Human Hair. 5th ed. Springer; 2012. Weathering of the cuticle, and damage from heat, ultraviolet light and wet combing.

None of these studies involve Radico or plant colour. They describe how light and wear behave on hair in general.

Radico Colour Me Organic is a cosmetic product for external use on hair. Patch test 48 hours before every new shade. Nothing on this page is medical advice.