Two Hour Henna Tattoo. Henna is a natural dye, which when mixed into a paste and applied to skin, hair and nails, has a staining quality. Fresh henna in powder form is green. Once it is mixed into a paste and applied to the skin, the paste turns black, which you can see in some of the photos. After the paste is washed off, the remaining stain ranges in color from light orange to dark burgundy, which you can see in other photos. The stain can last up to 3 weeks. The art of decorating the skin with henna is called Mehndi.