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Hair coloring, or hair dyeing, is the practice of changing the hair color. The main reasons for this are cosmetic: to cover gray or white 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 coloring can be done professionally by a hairdresser or independently at home. Today, hair coloring is very popular, with 75% of women and 18% of men living in Copenhagen having reported using hair dye (according to a study by the University of Copenhagen). At-home coloring in the United States reached $1.9 billion in 2011 and was expected to rise to $2.2 billion by 2016.

Application techniques
Off-scalp

Hair with blonde highlights
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, but not all work on top of a single base color. These are referred to as:

Highlighting, where sections of hair are treated with lighteners
Lowlighting, where sections of hair are treated with darker hair colors
Splashlighting a horizontal band of bleached hair from ear to ear
There are also newer coloring techniques such as ombré, shatush, balayage, airtouch, in which hair is dark on the crown and bit by bit becomes lighter toward the ends.

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 the hair to be colored. Employed especially when more than one color is to be applied, this method ensures that only the desired hair strands are colored, and the rest spared.
Cap, when a plastic cap is placed tightly on the head and strands are pulled through with a hook, a method infrequently practiced other than for applying highlights to short hair.
Balayage, where hair color is painted directly onto sections of the hair with no foils used to keep the color contained, a method growing in popularity due to its observed effect of appearing more natural. The difference between balayage and ombré is that a balayage requires more precise hand-painting sections of hair and typically costs more.
Dipping or tip dyeing, similar to balayage in that the color is painted directly on the hair, resulting in a more uniform color coverage on hair ends.
All coloring techniques can be used with any type of color. For lightening, the hair sometimes has to be bleached before coloring.

On-scalp

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 nearest the scalp) Root touch-ups are repeated every 4–6 weeks as the natural color grows in and becomes apparent. People who color their hair to disguise gray often have these root touch-ups.
All-over color, where the person wants all of their hair to be a different solid color
Block coloring, where the person wants two or more colors applied to their hair, resulting in dimension and contrast.

Permanent
Permanent hair color generally contains ammonia and must be mixed with a developer or oxidizing agent in order to permanently change hair color. Ammonia is used in permanent hair color to open the cuticle layer so that the developer and colorants together can penetrate into the cortex.

Demi-permanent

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.

Semi-permanent
Semi-permanent hair coloring involves no developer (hydrogen peroxide) or ammonia, and is thus less damaging to hair strands.

Semi-permanents may still contain the suspected carcinogen p-phenylenediamine (PPD) or other related colorants.

Temporary color
Temporary hair color is available in various forms including rinses, shampoos, gels, sprays, and foams.