As in the case of hair dying, different shades of brown, maroon and black henna are available for skin.  However, each person reacts chemically to the henna in a unique way.  Factors include body heat and acidity, pore size and environment.  Heat seems to help.  Bake the design in the sun or take a hair drier to it.

Mehndi body art traditionally is done on hands and feet for a very practical reason:  that’s where the dye takes best.  The further away from the extremities you go, the lighter it will be.  The henna is brightest and lasts longest on the palms of the hands and soles of the feet.  These pores, called glass pores, have no melanin nor hair follicles to take up space.  On the top of the hand, there exists a visible change in color at the base of the fingers (brightest), at the wrist, and at the elbow (lightest).  Clue:  Upper armbands don’t get very dark.  Mehndi seems to last longest on feet and legs, and if the nails are stained, they will remain so until they grow out (yish...).

Black henna that works seems to be tricky to source.  Many Mehndi artists report muddy results and most black henna available at Indian stores washes right out.  They only use red and brown shades, while Northern Africa uses the black  for skin dyeing.  If you want the black, seek an Arab source.  However, beware of pre-mixed concoctions coming from abroad.  Reports have indicated that some recipes call for tar, lighter fluid and even battery acid.  Lovely.

Commercial dyes in vibrant colors are available through the internet and trendy stores that sell henna products.  However, they may not last long.

Some allergic reactions have been reported in using the henna, especially with the black henna.