How does henna create an orange-red colour on hair and skin?

Henna, derived from the henna tree (Lawsonia inermis), creates its characteristic orange-red colour through a specific chemical interaction. The leaves of the plant contain a substance that reacts directly with the keratin found in human hair and skin to form a bright pigment.1

This active dyeing agent is known as lawsone (2-hydroxy-1,4-naphthoquinone), a yellow crystalline dye. When applied, this compound binds to the keratin proteins, resulting in the lasting orange-red stain historically used for body art and hair colouring. Today, chemically altered versions of this substance serve as the foundation for a wide variety of commercial hair colourants.21