Permanent hair colouring agents, which don’t wash out, make up the vast majority of the market. Gradual (or progressive) hair dye is applied over a period of time to achieve a desired shade. Temporary colour can be washed out in the next shampooing, and semi-permanent or demi-permanent agents stay through multiple washes.

Permanent hair-colouring agents chemically alter and bind to hair, weakening the hair shaft and making it dryer and more brittle. This damage can be minimised by using conditioning shampoos or conditioners that contain silicone.

Bleaching your hair (such as with hydrogen peroxide) removes colour, leaving it white or yellowish white. This process shouldn’t make your hair fall out in large patches, but it can make the hair dry and brittle, just as dyeing it can. However, new hair growth isn’t affected by the bleach, so it will be healthy.

Frequent colour changes may cause irreversible damage to your hair, including split ends and dry 'weathering'. To avoid this, wait at least four weeks between colourings. To maintain the colour change affected by permanent hair colour, you only need to re-dye the hair when new hair grows and the roots begin to show. Hair on the scalp grows about 0.3mm to 0.4mm per day, so it would grow only 8.5 mm to 11 mm (a little more than a third of an inch) in four weeks.

Damage to colour-treated hair can be prevented by limiting exposure to water and sunlight. Hair can become dry with frequent washing, and sunlight can cause fading.

Products used for colouring the hair on your head should never be used on eyebrows (or eyelashes) because the dyes can cause serious reactions when put in direct contact with the eye. Eyebrow dyes or tints must also be in compliance with EU regulations. In general, you shouldn’t try to match the colour of your eyebrows to your hair colour, but if you’d like to darken them, you can use an eyebrow pencil, a powder or gel specifically made for eyebrows.

Our skin becomes more transparent as we age, so hair that’s too dark can make you look pale and hair that’s too light can wash out your complexion. For a natural look, some experts recommend that brunettes go one shade lighter than their base colour and blondes go one shade darker.

Permanent hair colouring generally works best to colour hair that is more than 50% grey. Grey hair has reduced pigment (the substance that gives your hair colour); the permanent hair colouring process removes the pigment from your natural hair so when the artificial colour is added to both the grey hair and the natural hair, you get a uniform, more natural look.

50% of British women use some type of hair dye, according to a 2010 survey of 1,000 men and women for a hair products manufacturer. The motivation for colouring hair is different for each individual and can include a desire to make a fashion statement or add some excitement to life or cover grey hair.

A 2010 survey found the number of men who have their hair coloured or highlighted in salons has risen by 50% since 2008.

More than 5,000 different chemicals are used in various hair colouring products. The chemicals used depend on the type of colouring agent and the colour change that’s desired. In 2006 the EU Commission announced a reassuring move for consumers, saying it would ban 22 different hair dye chemicals that it did not consider safe.

p-Phenylenediamine (PPD) is an organic compound used in over 99% of all permanent hair dyes. It is a known irritant and is illegal for use in eyeliners, mascara and other cosmetics. Some temporary colourants are PPD free, but virtually all semi-permanent and permanent colourants contain the ingredient. The EU’s view is that PPD is safe for use in hair dye but it is strictly regulated. In 2011, the EU reduced legal PPD levels to a maximum of 2% when the mixed dye is applied to the hair.

Before colouring your hair, Allergy UK recommends rubbing a bit of the dye on the inside of your elbow or behind your ear and waiting 48 hours to see if a rash develops. This is called a patch test, and you should do one each time you colour your hair to make sure you’re not allergic to that specific dye.

Henna is a natural dye made from the leaves and roots of the mignonette tree. It’s a semi-permanent colouring agent.

Henna typically produces a brown, orange-brown, or reddish-brown tint, although other ingredients can be added to it to produce other colours, such as black and blue.

Gradual, or progressive, hair dyes are typically used by men to colour grey hair. Some of these gradual rinses contain lead acetate, which is classified as a “probable human carcinogen", based on lab tests on animals. However, a review of lead acetate in hair colouring, concluded that the tiny amount of exposure to the substance was “toxicologically insignificant” and these products can be used safely. Be sure to follow the product’s instructions carefully and keep the products away from children.

A pooled analysis of four studies showed that women who started using hair dye before 1980 may be at increased risk of non-Hodgkin lymphoma than those who had not dyed their hair. Manufacturers changed the formulas for their dye products in the mid- to late 1970s, but the research does not prove that hair colouring agents were to blame.

The NHS says most research shows it’s safe to colour your hair while pregnant.  However, it says many women decide to wait to dye their hair until after the first 12 weeks of pregnancy as a precaution. If you’re pregnant and concerned about colouring your hair, you may want to consider highlights instead because fewer chemicals are involved.