Few traditions have the practice of squeezing a bit of lemon, tea or some essential oils to the paste in order to get a darker shade of mehendi and to increase the longevity of the stain. The reason mehendi stains darker on certain parts of the body is because of accumulation of keratin. Traditionally, ways to darken mehendi that have been passed on through generations is done by applying coconut oil or mustard oil on all the areas mehndi has been applied after removing the dried mehendi off of your hand.