The skin becomes one of various shades of brown. When mixed with mehndi oil, the powder can stain skin. Those tannins give the powder from the crushed leaves the ability to stain objects touched by the powder. Well, that powder happens to contain chemicals called tannins.
Why would someone want to put a mixture of oil and powder on the skin? If that powder is mixed with mehndi oil, it can be applied to the skin. Over the centuries, inhabitants of the countries in the Middle East have learned how to crush the henna leaves, and to thus produce a powder. In the warm climate of the Middle East, the heena plant, which has the scientific name Lawsonia inernis, grows beautifully.