How does henna affect hair that has already been coloured?

Henna interacts with the pigments already present in the hair structure, so the final shade on coloured hair is harder to predict than on natural hair. This applies especially to hair treated with chemical dyes, lighteners or toners: the reaction can give an unexpected result — anything from a shift towards red or copper tones to uneven colouring.

Before using it on coloured hair for the first time, it is advisable to cut off a small strand and apply the prepared mixture to it, leaving it on for as long as you plan to for the main colouring. This lets you see the actual result without risking the whole length.

It is also important to bear in mind that henna is a long-lasting dye. Lawsonia inermis binds to the surface of the hair, which makes it harder to apply a chemical dye over it later: unpredictable reactions are possible. If you plan to return to chemical colouring in the near future, take this into account in advance.