The hair colour aisle is a masterclass in confusing terminology. Semi-permanent, demi-permanent, temporary, permanent, and a category called “washes out in 28 shampoos” which is a claim nobody has ever verified.

The distinctions matter, though, because they determine how much damage you’re doing, how it fades, and whether you’ll still be living with the decision in a year. Where each fits in a maintenance routine is covered in the hair colour care guide; this is about choosing between them.

The three categories, properly explained

Semi-permanent

No developer, no ammonia, no lifting. The pigment coats the outside of the hair shaft and sits partially in the outer cuticle layers. Nothing penetrates the cortex, and the hair’s own structure isn’t chemically altered.

Lasts: 4 to 12 washes, realistically. The “up to 28 washes” on many boxes assumes infrequent washing, cool water and no heat styling.
Can it lighten? No. It can only add colour, so it will never make hair lighter than it already is.
Grey coverage: partial. It tints grey rather than covering it, giving a blended, highlighted effect rather than solid coverage.
Damage: minimal. This is the gentlest real colour option.

Demi-permanent

Uses a low-volume developer, which lifts the cuticle slightly so pigment can sit just inside it. No ammonia, so no significant lightening.

Lasts: 20 to 28 washes, fading gradually.
Can it lighten? Barely. It can deepen, tone and blend.
Grey coverage: good — up to about 50% grey coverage reliably.
Damage: low to moderate.

Permanent

Ammonia or an alkaline agent swells the cuticle fully open, existing pigment is removed, and new pigment is deposited in the cortex where it stays.

Lasts: until it grows out.
Can it lighten? Yes.
Grey coverage: full.
Damage: the most of the three, and the effect is cumulative over years.

What actually determines how long semi lasts

The number on the box is close to meaningless because five variables swamp it.

Porosity. The big one. Damaged, bleached or previously coloured hair grabs pigment and holds it — semi-permanent on bleached ends can last months and occasionally never fully leaves. Healthy virgin hair barely holds it at all.

Wash frequency. Every wash removes pigment. Washing daily can strip a semi in under a week.

Water temperature. Hot water lifts the cuticle and pigment escapes. Cool rinses meaningfully extend the life.

The shade. Red fades fastest — the molecules are largest and least stable. Blue and green go next. Fashion pastels fade fastest of all. Browns and blacks last longest.

Heat styling. Straightening daily drives pigment out. This is the variable people underestimate most.

Who semi-permanent suits

Anyone who changes their mind. The single best reason to choose it. If you’re not certain about a shade, semi lets you find out for a month rather than a year.

Fashion colours. Almost all vivid pinks, blues, purples and greens are semi-permanent by nature. They sit on the surface and they were always going to fade.

Damaged or fragile hair. No developer means no further structural damage, which matters if your hair has already been through bleach.

People with early greys. The blended, tinted effect on grey often looks more natural than solid coverage, particularly as more grey comes through.

Anyone tired of a regrowth line. Semi fades from the ends rather than growing out from the roots, so there’s no hard demarcation to maintain.

Refreshing between salon visits. Though for pure tone maintenance a colour depositing conditioner is gentler still.

Who it doesn’t suit

Anyone wanting to go lighter — semi cannot lift, full stop.

Anyone with substantial grey who wants proper coverage. Semi tints; it doesn’t cover. You’ll get a softened, salt-and-pepper effect rather than uniform colour.

Anyone wanting a dramatic change. Going from dark brown to blonde requires lightening, and nothing without a developer can do that.

Anyone who wants results that last more than a few weeks without repeating the process.

Applying it at home

Semi-permanent is the most forgiving colour to do yourself, which is part of the appeal.

Do a patch test 48 hours before. Every time, even if you’ve used the product before — sensitisation to hair dye ingredients can develop suddenly, and reactions can be serious. This is not a step to skip because you’re in a hurry.

Start with clean, dry or towel-dried hair. Clarify first if you’ve got product build-up, because a coated strand won’t take pigment evenly.

Wear gloves. Put an old towel round your shoulders and Vaseline along the hairline to stop staining.

Apply in sections, mid-lengths and ends first, roots last — roots process faster because of scalp heat.

Follow the timing on the box, then rinse with cool water until it runs clear. Skip shampoo for the first rinse.

Then don’t wash again for 48 hours.

Making it last

Sulphate-free shampoo, always. Wash every third day rather than daily. Cool water. Deep condition weekly. Keep heat styling to a minimum, and use a heat protectant spray when you do. Avoid chlorine and use a swimming cap.

Top up with a matching depositing conditioner between applications rather than recolouring — it’s gentler and gives more control.

If you’re a blonde using semi to tone rather than to colour, note that purple shampoo and semi-permanent toner do overlapping jobs and stacking both can push the tone too far.

Removing it early

Usually straightforward, because the pigment is on the surface.

Clarifying shampoo two or three times, with a deep conditioner after each. Hot water helps here. A colour remover designed for direct dyes works for stubborn fashion shades.

The exception is bleached hair, where semi-permanent can stain the porous cortex more or less permanently — particularly blues, greens and reds. If you have very light bleached hair, understand before you apply a vivid shade that “washes out” may not fully apply to you.

A note on grey

If you’re deciding how to handle grey, semi-permanent is worth considering seriously and often gets dismissed too fast.

It blends rather than covers, so as more grey arrives, the transition is gradual rather than a monthly battle with a root line. It does no structural damage. And it can be stopped at any point without a growing-out phase.

The alternative, if you’re mainly bothered by the visible line rather than the grey itself, is a root touch up spray — cosmetic, same-day, and it removes the pressure to make a permanent decision at all.

Always patch test 48 hours before colouring, including with products you’ve used before. If you have ever reacted to hair dye, black henna or a temporary tattoo, speak to your GP before using any colour containing PPD.