Hair dyes vary greatly in terms of their permanence. They can mainly be divided into permanent, demi-permanent, semi-permanent and temporary dyes.
Permanent dyes generally contain ammonia and because they bleach the colour out of dark hair, only permanent dyes can achieve a look that’s lighter than the hair’s natural shade. These dyes achieve their permanence by opening the hair cuticle and forming large molecules which lodge inside the hair’s cortex, meaning they can’t be washed out. Consequently, the only way to get rid of permanent hair dye is to grow it out. Permanent hair dyes generally cause the most damage to hair and contain active chemicals in the largest amounts, but also offer by far the most grey hair coverage. A ‘tint’ is a permanent dye in one uniform colour.
Like permanent colour, demi-permanent products use a developer to activate the dye, but unlike permanents they contain no ammonia, which means that while they’re less damaging, they can’t lighten naturally dark hair. Because they don’t lift the colour from hair like a permanent dye, however, the final result can be less uniform - and consequently more natural-looking - than a permanent dye. A demi-permanent dye will typically wash out within around twenty-five shampoos.
It’s often thought that semi-permanent dyes contain no developer and work purely by coating the hair strand. However, this isn’t quite true - while these dyes do coat the strand, they generally also contain developer, albeit in smaller quantities than demi-permanent and permanent dyes, and do partially penetrate the hair strands. As such, it’s important for those with a PPD allergy not to assume a semi-permanent dye is safe, as many (though by no means all) of these dyes still contain PPD and function via oxidisation, the process which can trigger an allergic reaction. Semi-permanent dyes will cause less damage than demi- or permanent dyes, but generally only last for six to twelve washes. Bear in mind that despite the name, the colour will not entirely leave your hair, but fade. This will typically take longer on hair that has previously been lightened with a permanent colour.
Temporary dyes or ‘Direct dyes’ are often the best ‘party’ option, offering vibrant colours that wash out easily. These colourants are available in sprays, sachets, pots, powders and other formulas, and cause no structural change to the hair. The molecules in temporary dyes accumulate or ‘adsorb’ on the hair’s surface, and can only penetrate the cuticle if the hair is particularly damaged or dry. Generally, these dyes will generally wash out with a single shampooing, though they may last longer in hair that has previously been lightened with a permanent colour. Many dyed blondes use temporary or direct dyes to add silver, grey, blue or lilac tones to bleached hair.