What colour will it be?

Henna has only one dye molecule that stains the skin only one colour. That colour will appear different from orange - reddish brown - dark mocha - black cherry.  Why the variation on one colour?

Thickness of skin: the more layers of stained skin, the darker the henna will appear (think a folded sheer material).   This is why the fingertips can become a dark black cherry colour.
Following instructions: it takes 6 - 8 hrs for the henna dye to penetrate all available layers of dead skin cells. Don't pick off your henna paste early!
Oxidation: the stain will always be light at first; it darkens over 2 days with oxidation.
Personal body chemistry: hormones, pH levels temperature all affect stain.