Why plant colour stays
Five Pigments.
Four Ways of Staying.
Nothing in Radico needs the cuticle opened to get in. But how each plant then stays on the hair is a different story — and once you know which pigment does what, the green stage, the 72-hour wait, the fading order and the no-hairdryer rule all follow from it.
Four ways of staying, on one fibre. The cuticle is the overlapping outer layer of a strand. Henna bonds into it; indigo forms inside it and is trapped; manjistha and hibiscus rest on it; turmeric stains it. None of them needs it prised open.
| Pigment | From | How it stays | What that means | How well it is known |
|---|---|---|---|---|
| Lawsone | Henna leaf Lawsonia inermis | Bonds to keratin — a covalent bond, the same kind that holds a protein together | Wears off with the hair rather than washing out. The most wash-fast colour in the range | Measured on hair |
| Indigo | Indigo leaf Indigofera tinctoria | Arrives colourless and soluble, turns blue in air, and is trapped where it formed | Keeps developing for two to three days; needs the hair damp; fades sooner than henna | Measured on hair |
| Anthraquinones | Manjistha root Rubia cordifolia | Rests on the surface, held by weak attractions. In textiles these dyes need a metal mordant; hair gets none | Warms and deepens a red while it lasts. Washes down over time | Not measured on hair |
| Anthocyanins | Hibiscus flower Hibiscus rosa-sinensis | Adsorbs onto the surface without bonding; the colour itself shifts with pH | A cool red beside a warm one. Semi-permanent | Measured — related pigment |
| Curcumin | Turmeric root Curcuma longa | Already coloured; stains the surface directly with no development step | What you see at the rinse is what you keep. Fades fastest, especially in sunlight | Molecule well known |
Where this page sits. Our How It Works page explains what a blended shade does on your head and why results go wrong. This page goes one level down: what each plant is actually doing, on its own, and how sure anyone is about it. The two agree with each other; this one just has more plants in it.
One correction to a phrase we have used elsewhere: only one of these pigments is “already coloured” when it goes on. Turmeric is. Henna and indigo both arrive as colourless precursors and become pigment on the hair. What all five share is narrower — none of them needs the cuticle opened.
Plant by plant
What Each One Is Actually Doing
Henna
Lawsonia inermis
BondsIn the plant
Almost no free pigment. The leaf stores hennosides — lawsone with a sugar attached, which keeps it colourless and stable while the leaf is alive.
Getting out
Water wakes the leaf's own enzyme, a β-glucosidase, which cuts the sugar off. What is left oxidises into lawsone — the orange-red you see in the paste.
Staying on the hair
Lawsone is reactive. It joins keratin by Michael addition: a covalent bond to the protein's own side chains, the same kind of bond that holds the protein together.
This is the only one of the five where the pigment becomes part of the fibre. That is why a henna shade does not rinse out. It wears off, at the rate the outer layers of the hair wear off, which is slow.
One honesty about the bond, because our other pages lean on it. Not every lawsone molecule that lands on the hair makes that bond. Some sits loosely, held by weaker attractions, and that loosely held fraction is what an alkaline rinse can lift back out. The bonded fraction stays. So “henna is permanent” and “alkaline water fades henna” are both true, about different molecules on the same strand.
Dallmann N, Vill V, Straske F. Reinventing henna: enzyme-catalysed colour release from stabilized Lawsonia inermis L. extracts. Int J Cosmet Sci. 2026;48:310–326. doi:10.1111/ics.70029. The Michael addition of lawsone to keratin is the best-characterised binding mechanism of any plant hair colour.
Indigo
Indigofera tinctoria
Forms in placeIn the plant
No blue in the leaf at all. It holds indican — indoxyl with a sugar attached — which is colourless and dissolves in water.
Getting out
The same kind of enzyme cuts the sugar off, leaving indoxyl: still colourless, still soluble, and small enough to slip into the outer layers of the strand.
Staying on the hair
Air does the rest. Two indoxyl molecules join into indigo, which is blue and will not dissolve. It formed inside the hair, and now it cannot get back out.
Indigo never bonds to anything. It is held the way a pebble is held by a jar with a narrow neck: it got in as something smaller and became something larger. This is vat-dye chemistry — the same route that colours denim, where the dye is also formed inside the fibre rather than attached to it.
A side route matters too. Some indoxyl oxidises a step further into isatin, and isatin joining with indoxyl gives indirubin, which is red-violet rather than blue. That reaction is slower, so it arrives late — part of why the tone keeps moving for days.
Everything strange about indigo follows from this. It is green at the rinse because the reaction is halfway through. It keeps developing for two to three days because the reaction runs on. It stops if you blow-dry because the reaction needs water. It fades sooner than henna because nothing is holding it but the fibre's shape. And it holds better over henna — which we can see, but cannot yet explain in terms of mechanism.
Klaas S, Vill V, Straske F. Indirubin as a red hair colourant from Indigofera tinctoria L. Int J Cosmet Sci. 2025;47(5):877–886. doi:10.1111/ics.13076. Note that this study was run on yak hair and set out to control the colour shift, not to describe a product. The vat-dye mechanism itself is textile chemistry of long standing.
Manjistha
Rubia cordifolia
Sits on the surfaceIn the plant
Red anthraquinones — mainly purpurin and munjistin in this species, with some pseudopurpurin. The European madder, Rubia tinctorum, leans on alizarin instead.
Getting out
These are already coloured. Water carries them out of the root and onto the hair. There is no enzyme step and no development.
Staying on the hair
Here is the catch. In textiles, madder is a mordant dye: it needs a metal ion such as aluminium to lock it onto the fibre. Hair colour uses no mordant. What is left is weak attraction — hydrogen bonds and the like — to the surface.
So manjistha's red rests on the hair rather than joining it. It warms and deepens a shade for as long as it stays, and it washes down gradually. On dark hair very little of it is visible in any case; it does its work under the henna.
We are not going to dress this up. How much of manjistha's pigment actually ends up on a hair fibre, and how long it lasts there, has not been measured by anyone we can find. What we can tell you is what is in the root and how this class of dye behaves on other fibres.
Purpurin, munjistin and pseudopurpurin are well characterised as the colorants of R. cordifolia, and the behaviour of anthraquinone mordant dyes on textile fibres is standard dye chemistry. We have found no study measuring their uptake or fastness on human hair.
Hibiscus
Hibiscus rosa-sinensis
Sits on the surfaceIn the plant
Anthocyanins — the pigment family of blackcurrant, red cabbage and red wine. In this flower, mainly cyanidin with sugars attached.
Getting out
Already coloured and water-soluble. Out of the petal and onto the hair with nothing more than water. No enzyme, no development.
Staying on the hair
Anthocyanins adsorb onto keratin without bonding to it, and pile up on each other once there. The colour they show depends on pH: red in acid, purple towards neutral, blue in alkaline conditions.
Cyanidin reads magenta — a red with blue in it. That is what makes hibiscus a cool red beside henna's warm one, and why it sits in shades that need the warmth held in check.
The pH behaviour is not a footnote. Amla, which is in nearly every one of our shades, keeps the paste on the acid side — which is the side hibiscus reads red on. Take the acid away and the same pigment drifts purple.
How well is this known? The pigment class has been measured on hair, in a study of blackcurrant anthocyanins. It found adsorption rather than bonding, build-up through the pigments stacking on each other, and blue tones stabilised by the hair itself. Hibiscus is the same class, with a different sugar attached. Hibiscus specifically has not been measured, and the blackcurrant authors hold commercial interests in that work, which is worth knowing when you read it.
We have watched the pH behaviour happen, though we have not measured it. Hibiscus mixed with lemon juice and left an hour on a white swatch comes out red-purple; the same powder mixed with plain water comes out blue-purple. That is one pigment in two arrangements, and it is the clearest demonstration we have of why amla’s acidity matters in a blend. What it is not is a measurement: we did not put a meter in either paste, and we did not test how long either colour lasted. See both swatches →
Rose PM, Cantrill V, Benohoud M, Tidder A, Rayner CM, Blackburn RS. Application of anthocyanins from blackcurrant (Ribes nigrum L.) fruit waste as renewable hair dyes. J Agric Food Chem. 2018;66(26):6790–6798. doi:10.1021/acs.jafc.8b01044 · PMID 29808681. Blackcurrant, not hibiscus; same pigment class. Two authors declare interests in a company commercialising the work.
Turmeric
Curcuma longa
StainsIn the plant
Curcumin, and it is already yellow in the root. No precursor, no sugar to cut off. Of the five, this is the only one that goes on the hair as the pigment it will stay as.
Getting out
Curcumin is barely soluble in water and very fond of oily surfaces — which is what the outside of a hair is. It stains on contact.
Staying on the hair
It sits on the surface, held by that affinity. No bond, no development, no waiting. What you see when you rinse is what you have.
Turmeric is the pigment in exactly one of our shades, Sun Blonde, and it behaves like nothing else in the range. No green stage, no 72-hour wait, no rule about air-drying. The trade-off is at the other end: curcumin breaks down in light, and it fades faster in sunlight than any pigment we work with. No formulation gets around that.
It is also a pH indicator — yellow in acid and neutral conditions, red-brown once the pH climbs past about eight. Amla, in the same pouch, keeps it on the yellow side.
Curcumin, PubChem CID 969516. Its light-sensitivity and pH-dependent colour are well documented chemistry. We have found little on its uptake onto hair specifically, and we cite the molecule rather than pretend to a study we do not have.
Colourless henna
Cassia auriculata in the colours · Cassia obovata in the care powder
Barely coloursIn the plant
Two different plants share the name. The one in our shades is C. auriculata. It is a legume, unrelated to henna, and its leaf has no single characterised colorant.
Getting out
Water, as with the rest. Whatever weak pigment is present comes out with it. It is first on eleven of our ingredient lists, and every one of those is a light shade.
Staying on the hair
Mostly it does not stay, because mostly there is nothing to stay. Its job is to take up room so the herbs that do colour are working on a smaller share of the powder.
This is dilution, not chemistry, and it is what makes a blonde possible at all. A pale shade needs less pigment per gram, and the honest way to get there is a plant that barely tints rather than a filler that is not a plant.
The other Cassia — C. obovata, which we sell on its own — has one old study behind it: on bleached hair its components bound to cysteic acid, a group that oxidation creates in keratin, and on dark hair they did not bind at all. That study does not transfer to C. auriculata, and we do not use it as if it did.
For C. obovata: Forestier JP. A cosmetic senna, Cassia obovata: ‘neutral henna’. Int J Cosmet Sci. 1981;3(5):211–226. PMID 19469940. For C. auriculata, the species in our shades, we have found no equivalent.
Where on the strand, exactly
What Each Pigment Holds On To
The section above says three of these pigments “sit on the surface”. A fair objection: if they are only sitting there, why does the colour last a month rather than a wash? The answer is that none of them is paint. Here is what they are holding on to — with the parts that are measured kept separate from the parts we are inferring.
A keratin chain, and the six groups a pigment can attach to. Solid lines are bonds that have been measured. Dashed lines are attractions we infer from how these classes of molecule behave elsewhere. The rightmost group exists only in hair that has been bleached or permed.
| Pigment | Kind of hold | Most likely partner on keratin | Status |
|---|---|---|---|
| Lawsone | Covalent bond — a Michael addition onto the quinone ring | The thiol of cysteine first, the amine of lysine second. Not the carboxyl groups: they are the wrong kind of chemistry for this reaction | Bond measured partner inferred |
| Indigo | No bond at all | Nothing. It forms as an insoluble solid between and under the cuticle scales and is held by its own size | Mechanism established |
| Anthraquinones manjistha | Hydrogen bonds, stacking, and — if a metal ion is present — a chelate through it | The hydroxyls of serine, threonine and tyrosine; the aromatic rings; munjistin’s own acid group to a protonated amine; and via a metal, the carboxylates | Inferred |
| Anthocyanins hibiscus | Ionic attraction, hydrogen bonds, and stacking on each other | In acid it is a cation, and hair above pH 3.7 is net negative: the carboxylates of aspartate and glutamate, and the sulfonates bleaching leaves behind | Adsorption measured partner inferred |
Why “sitting on the surface” still lasts a month
Four things are true at once, and none of them is a chemical bond.
One
It is in the cuticle, not on it
An hour or two of wet contact swells the cuticle slightly. Pigment gets between and under the scales. When the hair dries the scales close on it — less like paint on a wall, more like soil under a fingernail.
Two
Once there, it does not dissolve
Indigo is insoluble. Stacked anthraquinones and aggregated anthocyanins are close to it. Water alone does not move them; each wash with a surfactant lifts a small fraction. That is what a slow fade is.
Three
Many weak holds at once
These are flat molecules with several hydroxyl groups, so each one makes many hydrogen bonds and contacts simultaneously. Any one is trivial. To remove the molecule you must break all of them at the same moment — the principle a burr uses on a sleeve.
Four
Layers protect layers
Anthocyanins in particular stack on one another as they arrive. The outer pigment shields the inner. The first wash reaches only the top.
This is also why the fading order is what it is. Henna is the one bonded pigment, so it does not wash out at all; it leaves with the hair. Indigo is held by shape, so it goes as the cuticle wears. The two reds are held by attraction, so they thin with each shampoo. Put a shade together from two of those and it will change tone as it ages — a brown drifts warm because the blue goes before the red.
And it is why amla cuts both ways for hibiscus. Acid keeps the anthocyanin in its red, positively charged form — but acid also protonates the carboxylates it would be attracted to, leaving fewer places to hold on. Redder, and less firmly held. A balance, like most of what amla does.
A hypothesis we can neither confirm nor rule out
Could the plants be supplying their own mordant? This came up while we were writing this page, and it deserves to be set out rather than left in a notebook.
In textile dyeing, anthraquinones like purpurin need a metal ion — usually aluminium — to fix them to a fibre. Hair colour uses no mordant. But there is a long tradition of plant mordants: the leaves of Symplocos, which accumulate aluminium, have been used for centuries in India and Indonesia to do exactly this job. So the idea that a plant powder might carry its own metal is not far-fetched. The chain of reasoning would run:
- Every plant powder carries some iron and aluminium — from the plant itself and from soil on the leaf.
- Amla is acidic, and acid mobilises those metals. Its ascorbic acid also reduces iron to its more soluble form.
- Dissolved metal binds to the carboxylates on keratin; purpurin chelates the metal. A mordant, arrived at by accident.
The competing reaction is just as plausible. Amla is rich in gallic acid and tannins, and gallic acid plus iron is the black complex of iron-gall ink. That is why anyone who has coloured with henna in iron-rich water has been warned about muddied results. So the same metal that might fix manjistha might also dull it. Which effect wins is a question of quantities, and we have not measured the quantities.
We could. The laboratory that runs our heavy-metal testing can measure iron, aluminium, calcium and magnesium in each powder by the same route. A swatch coloured in deionised water beside one in hard water would separate what the plant brings from what the tap brings. Until we have done that, this stays a hypothesis, and we are labelling it one.
What is measured here, and what is not
Measured: that lawsone forms a covalent bond with keratin; that indigo forms in place as an insoluble solid; that anthocyanins adsorb to hair without bonding, in layers, and that hair stabilises their blue form; that hair carries a net negative charge above about pH 3.7.
Inferred, from how these molecules behave on other proteins and fibres: which residue lawsone attacks first; every hold listed for the anthraquinones; that the anthocyanin’s partner is the carboxylate; the entire mordant hypothesis. We think each inference is sound. None of them has been checked on hair, by us or by anyone we can find.
Mechanism to experience
Why the Rules Are the Rules
Every instruction we give about plant colour comes from one of the four mechanisms above. Here is which one, so that none of it has to be taken on trust.
| What you see or what we ask | Which pigment | Why |
|---|---|---|
| Hair looks green at the rinse | Indigo | The reaction that makes blue is halfway through. Indoxyl is colourless; the first products are greenish; blue comes as oxidation finishes. Normal, and it clears. |
| Wait 72 hours before shampoo | Indigo, and the loose fraction of henna | Indigo is still forming for two to three days and needs the hair to hold some moisture. Alkaline shampoo also lifts the lawsone that has not bonded. Both reasons at once. |
| Air-dry, no hairdryer | Indigo | The reaction needs water. Dry the hair and it stops where it is, and re-wetting does not restart it — we tested that. |
| Shades without indigo settle in a day | Henna, manjistha, hibiscus, turmeric | None of these has a slow reaction running after the rinse. What is there at 24 hours is the result. |
| Henna shades last longest | Henna | Lawsone is bonded into the keratin. It leaves when the outer layers of the hair leave, which is slowly. |
| Ash and indigo-led shades soften sooner | Indigo | Indigo is trapped, not bonded. Wear on the cuticle releases it. |
| Sun Blonde fades fastest, and in sunlight | Turmeric | Curcumin breaks down in light. It is also only on the surface. |
| Amla is in 23 of 24 shades | Henna, indigo, hibiscus, turmeric | Its acidity helps henna release its pigment, keeps hibiscus on the red side of its pH range, and keeps curcumin yellow. It also hinders indigo — it is a balance, not a free lunch. |
| Alkaline water or soap fades a red | Henna | Not the bonded lawsone — the loosely held fraction. Most visible on the three shades where henna leads the list. |
| It cannot lighten hair | All of them | Nothing here removes melanin. Every mechanism on this page adds; none subtracts. |
Two things this explains that are easy to miss
Why a shade is a recipe, not a pigment
A brown is not a brown pigment. It is henna's bonded orange-red with indigo's trapped blue formed over it, two different mechanisms in the same strand. That is why they fade at different rates — and why a brown that has been on for months drifts warm as the indigo goes first and the henna stays.
Why the order on the pouch tells you so much
Ingredient lists run in order of quantity. Once you know that henna bonds and indigo is trapped, which of the two comes first tells you how a shade will wear: henna-led, it holds; indigo-led, it softens. That single fact is on every pouch we sell.
What nobody has measured, including us
- How much manjistha pigment ends up on hair, and how long it stays. We know what is in the root and how this class of dye behaves on textiles. Nobody has measured it on hair that we can find.
- Hibiscus itself. The pigment class has been measured, on blackcurrant. Hibiscus has not.
- Why indigo holds better over henna. Everyone who works with these plants sees it. The mechanism — whether lawsone-modified keratin traps indoxyl more readily, or something else — has not been established.
- What proportion of lawsone bonds versus sits loosely. The two-fraction picture is well supported; the split has not been quantified at cosmetic use levels.
- Which residue on keratin each pigment actually holds. Lawsone’s covalent bond is measured; whether it lands on cysteine or lysine first is inferred. Everything listed for the anthraquinones, and the anthocyanin’s partner, is inferred from how those molecules behave on other proteins — we set the reasoning out above, labelled as reasoning.
- Whether the plants supply their own mordant. Every powder carries some iron and aluminium; amla’s acid would mobilise it; purpurin chelates metals. That chain would help fix manjistha — or, through gallic acid, dull it. We have not measured the metal content of a single powder, so we cannot say which. It is on the list.
- Anything about Cassia auriculata on hair. The only hair study on a “colourless henna” is on the other species.
- The ratios. How much of each herb is in each shade is held by one person at our production facility. The order is public; the proportions are not.
References
Dallmann N, Vill V, Straske F. Reinventing henna: enzyme-catalysed colour release from stabilized Lawsonia inermis L. extracts. Int J Cosmet Sci. 2026;48:310–326. doi:10.1111/ics.70029
Klaas S, Vill V, Straske F. Indirubin as a red hair colourant from Indigofera tinctoria L. Int J Cosmet Sci. 2025;47(5):877–886. doi:10.1111/ics.13076 · PMID 40399588. Run on yak hair; its aim was to control the indigo colour shift.
Rose PM, Cantrill V, Benohoud M, Tidder A, Rayner CM, Blackburn RS. Application of anthocyanins from blackcurrant (Ribes nigrum L.) fruit waste as renewable hair dyes. J Agric Food Chem. 2018;66(26):6790–6798. doi:10.1021/acs.jafc.8b01044 · PMID 29808681. Blackcurrant anthocyanins, not hibiscus; two authors declare commercial interests.
Forestier JP. A cosmetic senna, Cassia obovata: ‘neutral henna’. Int J Cosmet Sci. 1981;3(5):211–226. doi:10.1111/j.1467-2494.1981.tb00284.x · PMID 19469940. C. obovata, not the C. auriculata in our shades.
Curcumin: PubChem CID 969516. Lawsone: PubChem CID 6755.
Our own swatch tests, referred to above: air-drying versus blow-drying at 1 and 72 hours, and water temperature at 68, 105 and 212 °F.