Hair coloring is the practice of changing the color of hair. The main reasons for this practice are cosmetic (e.g., to cover gray hair, to change to a color regarded as more fashionable or desirable, or to restore the original hair color after it has been discolored by hairdressing processes or sun bleaching). Hair dyeing, which is an ancient art, involves treatment of the hair with various chemical compounds. Today, hair coloring is immensely popular, with over 75 percent of American women dyeing their hair, and globally hair colorants are a rapidly growing over-$7 billion industry. 
- 1 Off-Scalp Application techniques
- 2 On-Scalp Application techniques
- 3 Types of hair coloring
- 4 Adverse effects of hair coloring
- 5 History
- 6 Plant-based dyes
- 7 EU Hair Dyes Directive
- 8 See also
- 9 References
Off-Scalp Application techniques
Hair color was traditionally applied to the hair as one overall color. The modern trend is to use several colors to produce streaks or gradations, not all work on top of a single base color. These are referred to as:
- Highlighting, where sections of hair are treated with lighteners, and they usually create blonde streaks.
- Lowlighting, where sections of hair are treated with darker hair color.
- There are also newer application techniques such as ombré, in which hair is dark on the crown and bit by bit becomes lighter toward the ends, and splashlights, in which a horizontal band of bleached hair stretches from ear to ear.
These are off-the-scalp techniques, and can be applied by the following methods:
- Foiling, where pieces of foil or plastic film are used to separate off the hair to be colored; especially when applying more than one color.This keeps the color only on the desired pieces of hair and protects the rest of the hair.
- Cap, when a plastic cap is placed tight on the head and strands are pulled through with a hook.This method is not frequently practiced anymore with the exception of short hair highlighting.
- Balayage, where hair color is painted directly onto sections of the hair with no foils used to keep the color contained. This method is growing in popularity because of it's ability to look more natural and less placed.
- Dipping or Tip Dying, similar to balayage in that the color is painted directly on the hair, this focuses on a more solid level of coverage on the ends of the hair. 
All application techniques can be used with any type of color. For lightening, the hair will sometimes have to be bleached before coloring.
On-Scalp Application techniques
Hair coloring can also be applied on the scalp for a more solid level of coverage
- Root Touch-Up, where color is applied only to the most recent section of re-growth. Usually the first inch of hair from the scalp. Generally those getting root touch-ups get this service repeated every 4-6 weeks as the natural color grows in and becomes apparent.
- All-Over Color, where the individual desires for all of their hair to be a different solid color.
- Block Coloring, where the individual wants 100% coverage but desires two or more colors to be placed resulting in dimension and contrast.
All application techniques can be used with any type of color. For lightening, the hair will sometimes have to be bleached before coloring.
Types of hair coloring
The four most common classifications are permanent, demi-permanent (sometimes called deposit only), semi-permanent, and temporary.
Permanent hair color
A very popular way to achieve permanent hair coloring is through the use of oxidation dyes. The ingredients of these products include 1,4-diaminobenzene (historically) or 2,5-diaminotoluene (currently), a coupling agent, and an oxidant. The process is typically performed under basic conditions.
The mechanism of oxidation dyes involves three steps: 1) Oxidation of 1,4-diaminobenzene derivative to the quinone state. 2) Reaction of this diimine with a coupler (more detail below). 3) Oxidation of the resulting compound to give the final dye.
The preparation (dye precursors) is in the leuco (which means colorless) form. Oxidizing agents are usually hydrogen peroxide, and the alkaline environment is usually provided by ammonia. The combination of hydrogen peroxide and the primary intermediate causes the natural hair to be lightened, providing a "blank canvas" for the dye. Ammonia opens the hair shaft pores so that the dye can actually bond with the hair and speeds up the reaction of the dye with the hair. Now people can dye their hair without bleaching it.
Various combinations of primary intermediates and couplers provide a spectrum of shades of hair colors. The primary intermediates are aromatic para compounds, such as 1,4-diaminobenzene or 4-aminophenol. The couplers are meta-substituted derivatives of aniline. They come in three major classes based on the color that they produce when they react with the primary intermediate.
- Blue couplers include 1,3-diaminobenzene and its derivatives.
- Red couplers include phenols and naphthols, such as 3-aminophenol (CAS#591-27-5), 5-amino-2-methylphenol (CAS#2835-95-2) and 1-naphthol (CAS#90-15-3). The combination of 2,5-diaminotoluene with the coupler 3-aminophenol gives a magenta-brown dye, while the combination of 2,5-diaminotoluene with the coupler 1-naphthol gives a purple dye.
- Yellow-green couplers include resorcinol, 4-chlororesorcinol, and benzodioxoles. These compounds produce broad-band absorption when they react to form dyes, allowing for more natural-looking hair colors. The combination of 2,5-diaminotoluene with the coupler resorcinol gives a greenish brown dye.
The first step shows the oxidation of p-phenylenediamine to the quinonediimine (C6H4(NH)2):
This species exists in equilibrium with the monoprotonated form (C6H4(NH)(NH2)+) (not shown). The second step involves the attack of this quinonediimine on the coupler. In organic chemistry, this reaction is called electrophilic aromatic substitution:
In the third and final step, the product from the quinonediimine-coupler reaction oxidizes to the final hair dye.
The resulting hair dye is also much larger than the precursor molecules, which causes the dye to bond to the hair.
Demi-permanent hair color
Demi-permanent hair color is hair color that contains an alkaline agent other than ammonia (e.g. ethanolamine, sodium carbonate) and, while always employed with a developer, the concentration of hydrogen peroxide in that developer may be lower than used with a permanent hair color. Since the alkaline agents employed in demi-permanent colors are less effective in removing the natural pigment of hair than ammonia these products provide no lightening of hair's color during dyeing. As the result, they cannot color hair to a lighter shade than it was before dyeing and are less damaging to hair than their permanent counterpart.
Demi-permanents are much more effective at covering gray hair than semi-permanents, but less so than permanents.
Demi-permanents have several advantages as compared with permanent color. Because there is essentially no lifting (i.e., removal) of natural hair color, the final color is less uniform/homogeneous than a permanent and therefore more natural looking; they are gentler on hair and therefore safer, especially for damaged hair; and they wash out over time (typically 20 to 28 shampoos), so root regrowth is less noticeable and if a change of color is desired, it is easier to achieve. Demi-permanent hair colors are not permanent but the darker shades in particular may persist longer than indicated on the packet.
Semi-permanent hair dye
Semi-permanent hair color has smaller molecules than temporary dyes. These dyes only partially penetrate the hair shaft. For this reason, the color will survive repeated washing, typically 4–5 shampoos or a few weeks. Semi-permanents contain no, or very low levels of developer, peroxide or ammonia, and are therefore safer for damaged or fragile hair. However, semi-permanents may still contain the possibly carcinogenic compound p-phenylenediamine or other such ingredients. The U.S. Environmental Protection Agency reported that in rats and mice chronically exposed to PPD in their diet, it simply depressed body weights, and no other clinical signs of toxicity were observed in several studies.
The final color of each strand of hair will depend on its original color and porosity, so there will be subtle variations in shade across the whole head. This gives a more natural result than the solid, all over color of a permanent color. However, it also means that gray or white hairs will not appear as the same shade as the rest of the hair. If there are only a few grey/white hairs, the effect will usually be enough for them to blend in, but as the gray spreads, there will come a point where it will not be disguised as well. In this case, the move to permanent color can sometimes be delayed by using the semi-permanent as a base and adding highlights.
Semi-permanent color cannot lighten the hair.
Temporary hair color
Temporary hair color is available in various forms including rinses, shampoos, gels, sprays, and foams. Temporary hair color is typically brighter and more vibrant than semi-permanent and permanent hair color. It is most often used to color hair for special occasions such as costume parties and Halloween.
The pigment molecules in temporary hair color are large and cannot penetrate the cuticle layer. The color particles remain adsorbed (closely adherent) to the hair shaft and are easily removed with a single shampooing. Temporary hair color can persist on hair that is excessively dry or damaged in a way that allows for migration of the pigment to the interior of the hair shaft.
Alternative hair color
Alternative hair coloring products are designed to create hair colors not typically found in nature. The available colors are diverse. For example, the colors green and fuchsia.
Permanent alternatives in some colors are available from big haircare brands. Some alternative color shades are blacklight-reactive, to show up under nightclub lighting.
The chemical formulae of alternative color dyes typically contain only tint and have no developer. This means that they will only create the bright color of the packet if they are applied to light blond hair. People with darker hair (medium brown to black) need to use a bleaching kit before tint application. Some people with fair hair may benefit from prior bleaching as well. Gold, yellow and orange undertones in hair that has not been lightened enough can adversely affect results, especially with pinks, blues and greens. Although some alternative colors are semi-permanent, such as blue and purple, it could take several months to fully wash the color from bleached or pre-lightened hair.
Adverse effects of hair coloring
Hair coloring involves the use of chemicals capable of removing, replacing and/or covering up pigments naturally found inside the hair shaft. Use of these chemicals can result in a range of adverse effects, including temporary skin irritation and allergy, hair breakage, skin discoloration and unexpected hair color results.
Skin irritation and allergy
In certain individuals, the use of hair coloring can result in allergic reactions and/or skin irritation. Symptoms of these reactions can include redness, sores, itching, burning sensation and discomfort. Symptoms will sometimes not be apparent immediately following the application and processing of the tint, but can also arise after hours or even a day later.
To help prevent or limit allergic reactions, the majority of hair color products recommend that the client conduct a patch test before using the product. This involves mixing a small quantity of tint preparation and applying it directly to the skin for a period of 48 hours. If irritation develops, manufacturers recommend that the client not use the product.
European dermatologists have, however, strongly advised against such pre-use testing, as it entails additional sensitisation (allergy) risk and the interpretation by lay people may not be sufficiently accurate Thyssen et al. 2012.
Hair that has been damaged by excessive exposure to chemicals is considered over-processed. This results in dry, rough and fragile hair. In extreme cases, the hair can be so damaged that it breaks off entirely. This is especially true for Afro-Caribbean hair, especially if used in combination with relaxers. Treatments are available  but the only real solution is to stop the use of chemicals until new hair grows and the damaged hair is cut off.
Skin and fingernails are made of the similar type of keratinized protein as hair. That means that drips, slips and extra hair tint around the hairline can result in patches of discolored skin. This is more common with darker hair colors and persons with dry absorbent skin.
This discoloration will disappear as the skin naturally renews itself and the top layer of skin is removed (typically takes a few days or at most a week). A good way to prevent dye discoloration is to put a thin layer of petroleum jelly or any oil-based preparation around the hairline. It is recommended that latex or nitrile gloves be worn to protect the hand.
Several factors influence the final color of the hair following the coloring process.
- For semi-permanent and demi-permanent color, the final color is a blend of the natural color of the hair and the dye color.
- Bleached hair will often require pre-pigmentation before a color application. Dyeing bleached hair brown can result in grey or very ashy (grey sheen) hair.
- Previously color treated hair can react unpredictably with subsequent color treatments.
- Previous use of shampoos which deposit a layer of plastic on the hair can block the action of the dye.
- Presence of minerals, salts, chlorine or other contaminants in the water used in the coloring process
- Certain prescription drugs can alter hair chemistry
- Coloring dark hair to achieve a desirable shade of blond requires bleaching, followed by a secondary color treatment. Bleached hair can still have a yellow or coppery shade. A violet-based color can cancel out yellow tones, and a blue-based shade will cancel out coppery orange.
- Porosity of hair can affect the final shade. Porous hair often absorbs more color, which sometimes makes darker than expected.
Professional hair colorists have a number of products to correct or deemphasize unintended hair coloring outcomes.
- The salt lead acetate (the active ingredient in gradual darkening products such as Grecian formula) is toxic if ingested.
- Articles link the development of some forms of cancer (including leukemia, non-Hodgkin's lymphoma, bladder cancer, blood cancer, and multiple myeloma) with use of hair color. More specifically, prolonged use of permanent dark hair dyes can double a person's risk of getting some types of blood cancer.
- In 2004 a known human carcinogen, 4-aminobiphenyl or 4-ABP, was found in some commercial hair dyes. However, evidence is limited and inconsistent for the link between cancer from hair dye.
The dyeing of hair is an ancient art. In ancient times, the dyes were obtained from plants. Some of the most well known are henna (Lawsonia inermis), indigo, Cassia obovata, senna, turmeric and amla. Others include katam, black walnut hulls, red ochre and leeks. In the 1661 book Eighteen Books of the Secrets of Art & Nature, various methods of coloring hair black, gold, green, red, yellow, and white are explained. The development of synthetic dyes for hair is traced to the 1860s discovery of the reactivity of PPD with air. Hair dyeing is now a multibillion dollar industry that involves the use of both plant-derived and synthetic dyes.
Henna is an orange dye commonly used as a deposit-only hair color whose active component, lawsone, binds to keratin. It is therefore considered semi-permanent to permanent, depending on a person's hair type. Most people will achieve a permanent color from henna, especially after the second dye. With repeated use the orange color builds up into red and then auburn. While "natural" henna is generally a red color, variations exist. These variations usually contain ingredients from other plants and even synthetic dyes.
Indigo is natural dye from a plant (Indigofera tinctoria, suffructicosa, or arrecta) that can be added to henna or layered on top of it to create brown to black colors in the hair. On the color wheel, henna is orange, and indigo is blue, so as complementaries, the two colors work together to create brown tones. Like henna, indigo may fade after one application, but it becomes permanent on the hair with repeated use.
Using a plant-based color such as henna can cause problems later when trying to do a perm or permanent hair color. Some store-bought henna contains metallic salts which reacts to hydrogen peroxide that is used in hair lightening. This may lead to unpredictable results, such as green or blue tones in the hair. Henna is a healthy way to color hair, as long as no metallic salts are used.
EU Hair Dyes Directive
Hair dyes are cosmetics in contact with the skin, containing chemical products which can be dangerous for health. This is why these products are highly regulated, and some substances which have been scientifically proven hazardous, in general or from a certain concentration or quantity are being officially and legally forbidden or restricted.
The European Union is particularly stringent when it comes to health regulations. In an effort to ensure that hair dyes contain only safe substances, the European Commission adopted the Directive 2012/21/EU to restrict the use of around 45 chemicals in hair dyes. The directive on dyes is part of a general and comprehensive set of regulations, the EU Cosmetics Directive 76/768/EC.
- Wecker, Johann Jacob; R. Read (1661). Eighteen books of the secrets of art & nature. p. 83.
- Brian, Marshall. "How Hair Coloring Works". How Stuff Works. Retrieved 7 October 2012.
- Wilson, Cynthia (6 August 2012). "Dip-Dye Hair Coloring Tutorial and Styles". Womenio. Retrieved 6 October 2012.
- “Hair Preparations,” Ullmann’s Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim (2006). doi:10.1002/14356007.a12_571.pub2
- p-Phenylenediamine, U.S. Environmental Protection Agency
- "White Hair Treatment, Let Natural Herbs Work On Your Scalp".
- FDA info concerning lead acetate
- Government reference for Grecian formula's ingredients
- "Does hair dye cause cancer?". Cancer Research UK. 20 July 2006. Retrieved 2007-07-29.
- DeNoon, Daniel J. (January 26, 2004). "Hair Dye Linked to Blood Cancer: Long-Term Use of Dark, Permanent Dye May Raise Lymphoma Risk". Web MD. Retrieved 2007-07-29.
- Hair Dye Linked to Blood Cancer. Webmd.com. Retrieved on 2010-12-21.
- Hair Dyes and Cancer Risk – National Cancer Institute. Cancer.gov (2009-10-09). Retrieved on 2010-12-21.
- Corbett, J. F. Hair Colorants: Chemistry and Toxicology; Micelle Press: Dorset, U.K., 1998; Thompson, R. H. "Naturally Occurring Quinones" Academic Press: New York, 1957.
- The History of Hair Dye
- Olivier J. X. Morel, Robert M. Christie "Current Trends in the Chemistry of Permanent Hair Dyeing" Chemical Reviews, 2011, volume 111, pp 2537–2561. doi:10.1021/cr1000145
- “Beware of the phosphates and other phosphorus compounds in detergents”, SGS SafeGuards, Retrieved 11/2012