There are many other things you can use that seem like them might be useful.  Some don't do anything.  Other's hurt you. Why? Most dye and pigment molecules are too large to breach a skin cell, and not shaped the right way to bind with the keratin.  Other molecules are very small, and will  breach skin cells, but once they're inside your body they make you sick.

Bad Ideas Group #1
Useless
Here are things that will NOT make henna stain darker:
India Ink
Magic Marker 
Grape Juice
Black Paint

Bad Ideas Group #2
Use with Caution
Here are two things things that will make henna darker but MAY HURT SOME PEOPLE!
Walnut leaves, powder, husk, or root.  Many people have nasty allergies to walnut.
Camphor may make you and your client dizzy, nauseous and intoxicated.

Bad Ideas Group #3
There are other things that will stain the skin but are DANGEROUS because they are coal tar dyes. 
These can cause severe skin reactions and are carcinogenic.
NEVER USE in Henna:
Commercial Hair Dye, black or any other color
Bigen Hair Dye
Kali Black Mehndi
Anything with para-phenylenediamine in the ingredients
Anything with negrosine in the ingredients
Any dye product with an ingredient that ends in -diamine
synthetic textile dye
anything with the word "AZO" or Azine Dye in it

Bad Ideas Group #4
There are solvents that will make henna darker but are DANGEROUS. 
These are carcinogenic, cause skin reactions, and can make you very sick just breathing the fumes.
NEVER USE in Henna:
Turpentine
Gasoline
Lighter Fluid
Dry Cleaning Fluid
Kerosene
Lamp Oil
Citronella Oil  

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