As a henna artist I spend a lot of time discussing where on the body my clients should consider having their artwork applied. Sometimes they come to me with very specific ideas of what they want and where, and sometimes they just want to get the best looking henna in the most pleasing location. Depending on why you're getting your henna you may want to consider where on your body your henna will be shown off to it's best advantage!
Not all body parts are created equal when it comes to henna! Some parts of the body will get darker stains than others... and here's why. While the henna paste is sitting on your skin the natural occurring dye molecules (lawsone) are sinking into the upper layers of your skin, the stratum corneum (these are the layers of skin that exfoliate off, that's why henna is temporary). In some parts of your body these layers of skin are very thick, like the soles of your feet and the palms of your hands, and other places are very thin, like your face, throat, and torso.
Henna is a translucent stain, I like to think of it as layers of colored cellophane. If you take two sheets of red cellophane and hold them up to the light, you'll see a nice red color, but if you take 10 sheets the color that you get will be darker, and richer. Henna works in a similar way, if you have only 6 layers of skin that the henna can dye it will be a much lighter stain than on an area that has upwards of 50 layers! On the palms and soles there is so much skin to soak up so much lawsone that even natural henna can look almost black, and on your torso where the skin is thinner it will be a lighter, more delicate brown.
As you move away from the hands and feet the skin tends to get thinner and the henna therefore tends to get lighter. You can often see a very clean demarkation of where the palm ends and the forearm begins in the color change of your mehndi designs
All of this is not to say that henna is only beautiful if it is dark in color, I find the subtle delicateness of a light henna stain to often be just as beautiful!
Many clients come to me wanting very specific body parts adorned: to cover scarring from a particularly upsetting injury or surgery, to get a Henna Crown to endow a head that's gone bald from chemo treatments with a bit of empowering beauty, or to receive Maternity Henna to celebrate the coming of a new member of the family. All of these reasons to get henna are very specific to the placement on the body and it's the art and the meaning imbued into it that has so much importance and the darkness of the stain is secondary. Both scalps and bellies tend to have thinner skin and the resulting stains are not as dark as on the hands, feet, arms, or legs, but as long as your expectations are appropriate you will get just as much joy from these placements.