On which hair will the result be more noticeable — light or dark?

Lawsonia inermis gives hair a warm natural shade — from golden to coppery red. On blonde and light brown hair this effect is more pronounced: the henna pigment shows up well on a light base, and the resulting shade is rich.

On dark hair the result is softer: henna does not lighten or cover dark pigment, so the change in colour will be less noticeable — more of a subtle copper or chestnut sheen in the light than a distinct change of tone. At the same time, the visual effect of thicker-looking, well-groomed hair remains regardless of the original colour.

If a particular shade matters to you, the only reliable way to judge the result in advance is to test it on a separate strand, leaving it on for the same time you plan for the main colouring.