Henna, Indigo, Cassia: The Plants Behind Vegetal Hair Colour

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Henna, Indigo, Cassia: The Plants Behind Plant-Based Hair Colour

Plant-based hair colour does not rely on recent technology. The plants used to dye hair and textile fibres have been known for millennia across South Asia, the Middle East and North Africa. What is new is the scientific understanding of the molecular mechanisms at work — and the formulation that allows predictable, reproducible results from a home application.

Tresse Paris colour is built on three core plants: henna, indigo and cassia, complemented by additional plants for their hair-care properties.

Henna — Lawsonia Inermis

Henna is the oldest known dye plant. Its leaves contain lawsone (2-hydroxy-1,4-naphthoquinone), a unique pigmenting molecule that bonds to keratin through covalent bonds — a strong, durable chemical attachment found in no other natural colorant.

Colouring mechanism: lawsone penetrates the hair cuticle and binds to the amine groups of keratin. This bond is irreversible: henna does not "wash out" — it fades progressively as the hair grows and the outermost layers of keratin are naturally shed.

Colours produced: pure henna gives a colour ranging from copper-orange to auburn-brown depending on the natural base. On white hair: orange to bright auburn. On dark brown or black hair: the surface effect is imperceptible, but shine and volume improve markedly.

Hair-care properties of henna:

  • Seals cuticle scales (smoothing effect)
  • Increases the apparent diameter of the hair strand (volume effect)
  • Reinforces the mechanical resistance of the fibre (anti-breakage)
  • Mildly astringent and antifungal

Warning about "black henna": genuine natural henna never produces a black colour. Products sold as "black henna" contain PPD (para-phenylenediamine) — a chemical allergen classified as a strong sensitiser. This is not plant-based colour. This is not henna.

Indigo — Indigofera Tinctoria

Indigo has been used for over 6,000 years as a textile dye (traditional denim owes its colour to indigo). In hair colouring, it plays a complementary and indispensable role alongside henna.

Mechanism: indigotin (the active pigment of indigo) produces blue-violet tones which, combined with the orange of henna, create by subtractive mixing the full range of browns, chestnuts and blacks. The complete range of human brown hair tones is reproduced by precise henna/indigo ratios:

  • Little indigo + lots of henna: light chestnut, hazelnut
  • Balanced henna/indigo: deep chestnut, chocolate brown
  • Little henna + lots of indigo: intense black, blue-black

Important particularity: indigo alone does not cover grey hairs effectively. It needs henna as a "mordant" — a substance that helps pigments bond to the fibre. This is the reason for the 2-step method: henna first (which anchors on the grey hair), then the henna-indigo blend (which delivers the final colour). Without this anchoring step, indigo coverage on grey is patchy and short-lived.

Cassia — Cassia Obovata (Senna Obtusifolia)

Cassia is often called "neutral henna" — a misleading term since it does not colour hair. Its lawsone content is negligible. It belongs to the legume family, not to the Lythraceae like henna.

Effects on hair:

  • On very light hair (white, blonde, very light chestnut): barely perceptible golden highlights.
  • On brown or dark hair: no visible colour effect whatsoever.
  • Conditioning effect: the anthraquinones in cassia nourish, soften and add shine to the fibre — a purely hair-care benefit that makes it a valuable addition to any plant colour blend.

Cassia is used in Tresse Paris formulas for its conditioning properties and its ability to "soften" the final result by balancing the more astringent effects of henna.

Complementary Plants

Amla — Phyllanthus Emblica (Emblica Officinalis)

The amla fruit (Indian gooseberry) is exceptionally rich in vitamin C and tannins. In plant colour formulas:

  • Intensifies the final colour by improving pigment fixation
  • Neutralises overly warm or red-orange tones from henna
  • Acidifies the blend (lowers pH), which seals the cuticle and improves anchorage
  • Antioxidant properties that protect the hair fibre from oxidative damage

Bhringraj — Eclipta Alba

A traditional Ayurvedic plant used to stimulate hair growth. In colour formulas:

  • Improves scalp microcirculation
  • Strengthens the hair bulb
  • Contributes to shine and fibre conditioning

Shikakai — Acacia Concinna

A natural cleansing plant whose mild saponins gently cleanse without stripping. It prepares the scalp to receive the pigments by removing build-up without disrupting the natural acid mantle.

Reading an INCI Label: Real vs Fake

The INCI list (International Nomenclature of Cosmetic Ingredients) is mandatory on every cosmetic sold in Europe. For a genuine certified plant-based colour:

What you must see:

  • Lawsonia Inermis (henna)
  • Indigofera Tinctoria (indigo)
  • Cassia Obovata or Senna Obtusifolia (cassia)
  • Optionally: Emblica Officinalis (amla), Eclipta Alba (bhringraj)

What you must NOT see:

  • p-Phenylenediamine or PPD — strong allergen, disguised chemical dye
  • Resorcinol — suspected endocrine disruptor
  • Hydrogen Peroxide or Ammonium Hydroxide — chemical agents from permanent colour
  • Sodium Sulfite or Potassium Persulfate — oxidising bleaching agents

COSMOS Organic certification goes further than simply reading the INCI: it guarantees that ingredients have been verified by an independent third-party body, that plants come from certified organic agriculture, and that no unauthorised synthetic compound is present. It is the highest level of guarantee available for a natural hair colour.

Why These Plants — and Not Others

Hundreds of dye plants exist worldwide (turmeric, safflower, weld, woad...). The henna-indigo-cassia combination has prevailed for practical reasons:

  • Colour range: from golden blonde to intense black, covering all browns — a palette as complete as chemical colour.
  • Lasting fixation: lawsone's covalent bond has no equivalent among natural colorants. It is genuinely permanent in the chemical sense.
  • Validated safety: millennia of traditional use, complemented by modern toxicological safety testing.
  • Hair compatibility: the strengthening properties of these plants on keratin are scientifically documented.

Our Recommendation

Choosing a plant-based colour means first choosing the plants it is made from. At Tresse Paris, powders are 100% plant-based, COSMOS Organic certified, free of PPD, ammonia and peroxide. What you put on your hair is exactly what is stated on the label — no more, no less. Explore the full Tresse Paris range and find your shade based on your natural starting point and the science behind each plant.

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