There are three green plant powders:
Neutral Henna, Red Henna and Black Henna. 
Only one of them is henna!

Neutral henna, a green powder that smells like freshly cut grass, is neither henna nor neutral.  It is Cassia obovata.  Cassia obovata contains anthraquinones, particuarly Chrysophanic acid, a remarkable anti-fungal, anti-microbial and anti-bacterial.  Cassia obovata has a golden dye molecule that will stain dull blonde and gray hair yellow.  It will help damaged hair, make hair full, glossy, healthy. 

Learn more about Cassia Obovata at: http://www.hennaforhair.com/faq/cassiaobovata.html

Red henna, a green powder that smells like hay, is Lawsonia inermis, commonly known as henna.  The leaves of the henna plant have a red-orange dye molecule, Lawsone, a napthaquinone.  Henna will stain your hair red-orange; but this stain is translucent and will combine with your natural color.  Body art quality henna has a much higher dye content than the henna usually sold for hair.  Henna is the best hair conditioner of all.  It will make your hair heavy, thick and silky.

Learn more about henna at http://www.hennapage.com/henna/encyclopedia/growing/

Just Lawsonia inermis.  This is the only plant that is actually “real henna”  So how did this get so confusing? 

Learn about the history of henna for hair: http://www.hennaforhair.com/history/

Lawsonia alba and Lawsonia spinoza are misleading older names for Lawsonia Inermis.  When henna is a small and immature plant, it has low dye content and is spineless; when mature, it develops spines and higher dye content. Henna plants undergo this change when they are 3 years old.  When western botanists saw juvenile and mature henna plants, they thought they were seeing two species, and gave them different botanical names.  Lawsonia also has different colors of flowers.  The plants with white flowers are sometimes called var. alba, but they are used for dye as are the plants with yellow, pink and red flowers.  Learn more about henna flowers HERE: http://www.hennapage.com/henna/encyclopedia/growing/flower.html .  You can see henna spines HERE: http://www.hennapage.com/henna/encyclopedia/growing/  .

Only henna leaves have dye, and the highest content is in the leaf petiole.  There is no dye in the bark, twigs, or rootstock of Lawsonia inermis, and certainly not different colors such as black.  The roots of henna are never harvested for dye, as henna is a small tree that is kept in production for many years.  Though henna is grown in many different countries, the henna dye molecule is always the same red orange.  The leaf’s dye content differs according to climate and soil conditions, so the dye saturation may differ, but henna is not black in one country and red in another country.                                     
 
Learn more about how henna is grown and processed:
http://www.hennapage.com/henna/encyclopedia/geography/
http://www.hennapage.com/henna/encyclopedia/geography/indiahenna.html
http://www.hennapage.com/henna/encyclopedia/growing/Pakistan/

Some Blonde, Brown, Auburn, Mahogany, and other “shades” of henna are mixes of amla, indigo, walnut, rhubarb, and Lawsonia, with other plant or synthetic dyes added, and may have metallic salts added. Many of these products have no henna whatsoever and are chemical dyes. Some commercial brands that claim to be 100% natural may include a bottle of “developer”; beware!  This is a completely bogus addition, as far as henna itself is concerned and is the biggest indicator that your product is NOT even close to being 100% pure henna!  The labeling on these products is often misleading, inaccurate, false, or entirely missing.  The quality is often very poor.
 
Learn more about what's in henna packaged for hair:  http://www.hennaforhair.com/science/whatsinit.html

Learn more about the wide range of hair colors with you can achieve by mixing your own natural dyes: http://www.hennaforhair.com/mixes/

Absolutely not.  A “boxed” hair color will lighten your hair about 2 shades depending on the brand and strength. Those of you brunettes out there that have tried to go blonde with a boxed drugstore hair color are probably familiar with the brassy orange it will give you when the developer does not have the strength to lighten properly!  In order to lighten hair’s natural color more than 2 shades or so, it must first be bleached, which cannot be done with pure plant products. You must use bleach and peroxide developer mix. If you have a chemical color that is extremely dark and want to remove it your hairdresser may recommend a color “stripper”. These are effective but very hard on the hair. There are several on the market (one such product is “effesol”) These must be used with extreme care on over processed hair and aren’t recommended at all on permed or chemically straightened hair. This is a strong product and should really be used by a professional to be done properly and with the least damage.

Amla is the fruit of the emblica officinalis tree, which is dried and processed for hair and skin products.  Amla paste is used as an exfoliating astringent facial scrub, and in conjunction with indigo for adding gloss and curl to black hair.

For more information on Amla, see: http://www.hennaforhair.com/faq/amla/

 No such thing.  This may be for several factual or speculative reasons, such as:

1) Some brands of henna contain metallic compounds that react with the ammonia activator in synthetic hair dyes.  These reactions yield disasters like frog butt green hair, fried and brittle hair, and in some rare cases, the combination of metals and a freshly done chemical job might melt the hair off your head. Many henna hair dye producers do NOT provide accurate and complete labeling on their products.  Other producers lie about what’s in their product, or do not know what they’re selling. If you have dyed your hair, use only body art quality from a reputable source to be sure you won’t get frog butt green fried hair!

2) Henna is not bad for your hair; it is a natural colorant and strengthener, and has all sorts of other benefits such as the reduction of dandruff, elimination of ringworm and head lice.

3) Many people feel that hairdressers are anti-henna, or that they won’t use henna due to a higher cost of materials. Chemical colors aren't really cheaper than doing henna, or always more time effective. Granted, some people can slap a 1-color process on and be out in 45 min. But many people are at the salon for as long or longer than a henna application would take. The products needed to do a chemical color are actually in most cases more expensive than what you need to do henna.

It's not that hairdressers are necessarily "anti-henna". They are mostly just uneducated in the use of it. Please keep in mind that if you frequent this site and have hennaed your hair you probably know much more about this subject than you hairdresser does! A lot of factors go into that. First off, most aren't trained in how it works, and many only know what they are taught about "beauty supply henna" which is indeed a chemical and CANNOT be mixed with real henna due to reactions from metallic salts used in the "fake" henna. When you think metallic salts, think of a range of products including Grecian Formula, we call them progressive colors. They learn to be scared of it based on info like that, and they well should be, as in the world of beauty supply products that is the type of henna product that they still see most often, and the type that will cause the most adverse reactions.

 
Beauticians are trained in color in a way that you assess people’s individual coloring and try to steer them the right way color wise. Every person is not meant to be a redhead; it's actually the hardest color to wear successfully.  Your client may want henna; she may love the red hair. But it isn't the best color choice for her. And we are trained to suggest the best color option for the clients based on their own natural personal coloring. And we have to try and guarantee a result, or at least a fairly accurate result range. Henna, even if you know how to do it properly has such a wide result range that that in itself makes it a hard choice in a professional setting. And as a hairdresser it is very hard to believe people when they tell you what has been done to their hair. Most just know that they have color, but don't know what kind, can't remember when etc., and that makes it risky to offer henna due to possible chemical reactions.

4) Anybody can henna their hair, its not difficult!  As long as you pay attention, there is no need to call in an expert.  It is time consuming, yes, but not difficult in any technical aspect.
 
The goal of Henna for Hair is to explore, research, and understand  natural hair dyes so people can achieve a wide color range with predicable results.  People in North Africa, the Middle East and South Asia have been doing this for centuries, but the mixes were unknown in the west, and often family secrets.   Henna, indigo, and cassia are ecologically sound crops for small land holders in marginal farming areas, and if the market for these products increases, it will benefit family farms and soils, as well as preserving our own health.

 Absolutely not.  If you previously had light blonde hair, and then hennaed it to a richer, darker color, you will definitely notice shedding hairs, but only because of the darker color and not because of any change stemming from the henna. Everyone sheds approx. 150 hairs per day. And you may loosen many more hairs while completing the process. These are just hairs that would have loosened into your hairbrush or shower drain. But henna cannot cause additional healthy hair to fall out.
On the other hand, you may actually shed somewhat less after hennaeing your hair several times. Lawsonia inermis contains tannin known as hennotannic acid.  Tannins are slightly astringent and their use will tighten the surface of the scalp and hair follicles, strengthening the follicles’ grasp on each hair.  Henna also penetrates the hair shaft, strengthens it, smooths the cuticle, thickens the hair, making your hair more resistant to breakage.

No, this is not true.  The Food & Drug Administration has rules about products imported into the United States if these products would be used on hair, skin, or in foods and drugs.  Henna is a plant material; as a natural product, it falls into a specific area regarding importation regulations.
True henna is considered to be so safe when used on hair, that it is certified as being exempt from FDA regulations, provided that it is intended for use solely as a hair coloring agent, follows proper labeling procedures, and conforms to requirements regarding adulterants and other incidental components (see Title 21: Food and Drugs
PART 73—LISTING OF COLOR ADDITIVES EXEMPT FROM CERTIFICATION
Subpart C—Cosmetics  § 73.2190   Henna. ).