Because it is natural you may apply it repeatedly. Repeated applications result in a richer color. It fades naturally within a few months so it is great if you don't want that hard line of roots in your natural hair color as it grows out. It's perfect for graying hair that you don't really want to alter or get rid of permanently (believe it or not, some of us actually like our gray hair). Plus, it's extremely inexpensive.
Warning: Henna is a powerful stain, protect any porous surfaces and the skin of your face and neck before you begin working with this product.
The process: Shampoo your hair to remove all product and moisturizers. Mix henna and apply to hair, cover your head with a shower cap to hold in the heat for--at least--one to two hours. Shampoo hair to remove all traces of Henna, then condition, dry and style as usual.