It is vital that you test the skin for allergies before painting henna art on anyone’s body. Most people are not allergic to the henna itself, but some folks are allergic to other ingredients. Test small areas by mixing a bit of powder with water and letting it sit for about an hour. Then test for richness of color and allergies by dabbing a bit of the mixture on the sole of the foot of the person you will be painting. Once it dries, scrape it off. If the person has an allergic reaction or if you think the color is too light, get different powder and start over.