Semi, demi and permanent colour differ by what they open

Both approaches to hair colour chemistry work. What differs is what they cost you, and the cost is what this sets out.

Semi-permanent colour

Semi-permanent dyes use relatively large pigment molecules that adhere to the outside of the strand and sit within the outer layers. There is no peroxide and no alkaline agent, so the cuticle is not deliberately opened and the natural pigment is untouched. The colour fades progressively with each wash as the molecules leave, typically over a handful of washes to a few weeks.

It can only add colour or darken, since nothing in the formula removes existing pigment from the hair. It is the least damaging category by a substantial margin, which is why it is the sensible place to experiment.

Demi-permanent colour

Demi-permanent formulas use a low-strength peroxide developer and generally omit ammonia in favour of a milder alkaliser. That opens the cuticle slightly, allowing smaller dye precursors to enter and combine into larger molecules inside the strand. The result lasts considerably longer than a semi-permanent, generally fading over many washes rather than washing out cleanly.

It deposits and can blend grey, and it does not lighten natural pigment in any meaningful way. For anyone wanting durable colour without significant lightening, this is usually the right compromise.

Permanent colour

Permanent colour uses an alkaline agent, commonly ammonia, to swell the hair and lift the cuticle scales properly open. A peroxide developer then both removes some natural pigment and drives the oxidation reaction that builds new colour molecules inside.

At the strengths actually sold, because those molecules are formed inside the cortex and are too large to escape, the colour does not wash out. It grows out instead, which is why permanent colour creates a visible root line and semi-permanent does not. The trade for permanence is that the process genuinely alters the hair structure every time it is performed.

Developer strength

Developers are sold in volumes that correspond to peroxide concentration, with ten volume around three per cent and twenty volume around six per cent. Higher volumes such as thirty and forty lift more natural pigment and cause correspondingly more damage to the strand.

The strength required depends on how much lift the target shade needs, which is why going lighter is always harder on hair. Using a stronger developer to speed up a process is the most common home colouring mistake with lasting consequences.

This is exactly the area where a professional assessment is worth the money, particularly on previously treated hair.

Why colour fades unevenly

Porous sections absorb more dye and release it faster, which is why previously bleached ends fade before mid-lengths. Red pigment molecules are among the largest and leave the fastest, which is why vivid reds are famously difficult to maintain. Hot water and strong detergents both accelerate loss by swelling the strand and lifting the cuticle.

After four to six weeks, sun exposure breaks down dye molecules directly, and coloured hair fades visibly over a bright summer. Colour-safe products are mostly gentler surfactant systems, which is a real if modest effect rather than a technology.

If you are pregnant, breastfeeding or using prescription treatment, check specific actives with a doctor or pharmacist before adding them.

Looking after coloured hair

Washing less frequently and at lower temperatures does more for colour retention than any specialist product. Protein and bond-building treatments address different things, and heavily processed hair often needs both at different times.

Heat styling on coloured hair compounds damage, since the strand is already structurally compromised. Overlapping permanent colour onto previously coloured lengths repeats the chemical process on hair that has already been through it. Any burning, itching or blistering of the scalp during colouring is a reaction that needs medical attention rather than perseverance.

Side by side

The takeaway

Choose the mildest category that achieves the shade you want, because permanence and damage come from the same chemistry.

A short routine you finish beats a long one you resent.