The best mehndi/henna paste in the world is homemade from fresh ingredients ... just like your grandmother's soup or bread!
The first ingredient is finely powdered leaf of the henna plant, lawsonia inermis. The fresh powder is olive green and smells a bit like hay or, in my opinion, matcha (powdered Japanese green tea).
The powder must be combined with an acidic liquid, and most artists choose lemon or lime juice. The acid in the juice breaks open the cellulose in the henna leaf powder, releasing the precious red dye for which mehndi is famous. Other acidic liquids that can be used include wine and vinegar ... but they don't smell nearly as nice.
This is henna paste at its most basic. I also like to add a saccharide or sugar such as dextrose (corn sugar), fructose (fruit sugar) or plain ol' table sugar (sucrose). This helps the paste become "stringy" and easier to drape. Sugar is also hydrophillic: it attracts water and helps the paste stay moist longer. The paste's dye is active only when the paste is moist.
The final ingredient in many pastes are essential oils of plants that have high levels of monoterpene alcohols -- referred to by those in the biz as "terps." Essential oils such as tea tree, cajeput, lavender and cypress act as a solvent to help release more of the dye to be available to stain the skin. They also smell delicious!
Other acceptable admixtures include culinary spices such as cardamom and fenugreek, floral waters such as rose water or orange blossom water, weak tea or coffee, and citric acid (lemon salt).
For a discussion of what should NOT be in henna paste, click here.
Now that you know what goes into a good, fresh paste, you can determine if any of the ingredients might be problematic for you. Most aren't, but anyone can be allergic to virtually any substance. Also, you can ask an artist what's in her paste and see if the ingredients sound good to you.
Who knows ... you might try to make some paste of your own!