Purple shampoo is the only hair product I know of that people use twice as often as they should because it works, and then blame when it stops working.

The pattern is always the same. Brassiness appears. Purple shampoo fixes it. Delighted, the person starts using it every wash. Three weeks later the hair is dull, slightly grey, faintly lilac at the ends, and feels like straw. And the conclusion is usually “this shampoo ruined my hair”, when what actually happened is that a fortnightly product was used fifteen times.

It belongs in every blonde’s routine — just less often than instinct suggests. Where it sits alongside everything else is covered in the guide to hair colour care.

Why blonde goes brassy

When hair is lightened, bleach removes pigment in stages. Dark hair doesn’t go from brown to white — it passes through red, then orange, then yellow, then pale yellow. Your colourist stops at the right stage and applies a toner to neutralise whatever warmth is left.

That toner is a deposited pigment, and it washes out. As it goes, the underlying warm tone in the hair — the orange and yellow that were always there — becomes visible again. That’s brassiness. It’s not new colour arriving; it’s the toner leaving.

Hard water, chlorine, sun and heat all accelerate it. Hard water in particular, because mineral deposits carry their own warm cast.

How purple fixes it

Colour theory. On the colour wheel, purple sits opposite yellow. Put one on top of the other and they cancel out towards neutral.

Purple shampoo contains violet pigment that deposits a small amount onto the hair surface with each wash, neutralising yellow tones and making the blonde read as cooler and cleaner.

Important limitation: purple only cancels yellow. If your hair is orange rather than yellow — which happens with darker natural bases, or when lightening hasn’t gone far enough — purple won’t touch it. Orange needs blue. That’s what blue shampoo is for, and it’s the product most brunettes-turned-blonde actually need.

Quick test: hold your hair against a white surface in daylight. Yellow tones, use purple. Orange tones, use blue.

How often

This is the whole question, and the answer is less than you think.

Once a week for most blondes. That’s the default.

Once every two weeks if your hair is fine, porous or very light — platinum and white blondes grab pigment fast and go lilac easily.

Twice a week only if you have stubborn brassiness, thick hair, or you’re in a hard water area, and only temporarily.

Never daily. The pigment accumulates faster than it washes out, and the formulas are generally more drying than normal shampoo.

Between purple washes, use your normal sulphate-free shampoo. Purple shampoo is a treatment, not your everyday cleanser.

How to use it properly

Wear gloves unless you want purple fingernails for a day.

Apply to wet hair, concentrating on the areas that go brassy first — usually the mid-lengths and ends, and the sections around the face that get the most sun. The roots on freshly coloured hair rarely need it.

Leave it on for two to three minutes to start. Not the ten minutes some bottles suggest. You can always leave it longer next time; you cannot un-tone hair that’s gone too far in one session.

Rinse thoroughly, then follow with a proper conditioner or mask, because purple shampoos are almost always drying.

If your hair is very porous — recently bleached, or damaged — halve the timing. Porous hair absorbs pigment at a completely different rate and this is where most over-toning accidents happen.

Fixing hair that’s gone too purple

It happens, and it’s usually reversible.

Wash two or three times with a clarifying shampoo, which strips the deposited pigment. A gentle dish soap works in an emergency, though it’s harsh and you’ll need to deep condition afterwards.

A lemon juice rinse — juice of one lemon in a jug of water, left on for a few minutes — helps lift residual pigment. Don’t sit in the sun with it on.

Then deep condition, and wait. Purple pigment sits on the surface rather than inside the strand, so it lifts within a few washes.

If the hair has gone genuinely grey or muddy rather than just lilac, and clarifying hasn’t shifted it after three washes, book in with your colourist rather than experimenting further.

Who shouldn’t use it

Natural blondes. Your hair isn’t chemically lightened and doesn’t have the same porosity. It’ll go dull rather than brighter.

Anyone with orange rather than yellow tones. Blue shampoo instead.

Brunettes. Purple does nothing on dark hair. Blue shampoo for brassy brunettes.

Anyone with warm-toned blonde who likes it. Honey, caramel and golden blondes are supposed to be warm. Purple shampoo will flatten exactly the tone you paid for.

Grey and silver hair. Actually a good candidate — purple shampoo neutralises the yellowing that grey hair develops from pollution, product and sun. Use it fortnightly.

When purple shampoo isn’t the answer

If brassiness returns within days of every wash, the underlying problem isn’t tone maintenance. It’s usually mineral build-up from hard water, and no amount of purple will out-run it. A chelating shampoo containing EDTA once a month is the actual fix, alongside a shower filter.

If your hair is fading in intensity rather than shifting in tone, purple won’t help either — a colour depositing conditioner replaces lost pigment in a way a toning shampoo doesn’t.

And if you’re mainly trying to stretch the gap between salon visits because of visible regrowth rather than tone, a root touch up spray solves a different problem entirely and does it in ninety seconds.

Keeping the dryness in check

Purple shampoos are drying by design — many contain sulphates to help the pigment deposit evenly, and bleached hair is already fragile.

Always follow with a mask or a rich conditioner. Deep condition weekly regardless. Use bond-building treatments every few weeks. And keep the heat down, because bleached hair has less structural integrity to lose.

If your blonde came from balayage rather than a full head, the maintenance differs slightly — the specifics are in balayage aftercare.