Put this down and go and look at the spill before you read any further. If it happened in the last ten minutes you can probably deal with it yourself with a roll of kitchen towel and cold water, and nothing I sell will improve on that. If it happened yesterday, read on, but adjust your expectations before you get to the end.

Hair dye is the one domestic spill where the clock does most of the work and the bottle does very little. That runs against everything the stain-remover aisle is built on, and it is the single most useful thing I can tell anyone about a landing carpet with a brown trail down it.

What is happening in the pile in the first ten minutes?

Permanent hair colour arrives in two parts and neither part is coloured when it lands.

In the tube you have dye precursors, small aromatic molecules such as p-phenylenediamine or toluene-2,5-diamine, along with couplers. In the bottle you have hydrogen peroxide at somewhere between three and twelve per cent, plus an alkali, usually ammonia or monoethanolamine. Mixed, the alkali swells the fibre, the peroxide oxidises the precursors, and those small molecules link up with the couplers into much larger coloured ones. In hair that reaction is what gives you colour that lasts. On a landing carpet the same reaction happens on the same timetable, twenty to forty-five minutes, and the difference is that nobody is rinsing it out at the end.

The first ten minutes matter because the molecules are still small and still water-soluble. Blot and rinse in that window and most of what you remove comes away before it has a chance to grow into something the fibre cannot release. Leave it forty minutes and you are asking a technician to take large polymerised molecules back out of the inside of a yarn, which is a different order of problem and often a failure.

Why the puddle looks weaker than the stain becomes

People relax when they see the spill. A fresh drip of mixed colour on beige carpet often looks watery, greyish, almost innocuous, which is why the household finishes the roots first and deals with the landing after. It darkens as it develops, exactly as intended, and by the time it looks alarming the reaction is most of the way done. The first ten minutes are spent watching something that does not yet look like a problem.

Does the kind of dye change what you can do?

Considerably, and knowing which one is on the floor should come before anyone touches it.

Oxidative permanent colour behaves as described above. Semi-permanent direct dyes, the bright ones in tubs, are a different animal. They arrive already coloured and many of them are cationic, and wool and nylon carry anionic sites that pull cationic dye in on contact. There is no twenty-minute window with those. A drip of vivid magenta on a wool landing is fixed in seconds, which is why I price a Directions-type spill and a box-colour spill quite differently on the phone.

Henna is the third category and the worst. It is a plant dye that binds to keratin, and wool is keratin. Henna on a wool carpet is chemically the same event as henna on hair, and it behaves accordingly: permanent, unresponsive to reducing agents, and beyond anything I can do except disguise. Compound hennas with metallic salts in them are worse again, because the salts can react with a peroxide-based treatment and cook the fibre, and I will not put an oxidiser anywhere near a henna spill for that reason.

The one that catches people out

Bleach kits and lightening powders drip too, and they leave no colour at all on the day. The householder wipes up a colourless smear, thinks nothing more about it, and finds an orange patch on the landing a week later when the peroxide has finished working on the carpet’s own dye. I have been called to a Peckham flat for what the tenant described as “a stain that appeared by itself”, and it was the previous Saturday’s bleaching session showing up on schedule.

What should you do before the developer finishes its work?

Scoop, blot, rinse cold, repeat, and keep your hands off it.

Lift the bulk with the edge of a spoon or a stiff card and get it off the carpet rather than into it. Then white kitchen towel or a white cloth, pressed straight down and lifted straight up, no circles and no scrubbing, because rubbing drives the liquid to the base of the yarn where extraction cannot follow. Change to a clean part of the cloth every single press. When the transfer slows, mist with cold water from a spray bottle, wait thirty seconds and blot again. Twenty repetitions of that, inside the window, will take a permanent-colour spill down to a shadow.

Cold water, not warm. Heat accelerates the same oxidation you are trying to interrupt. The other thing to avoid is any proprietary oxidising stain remover at this stage, because you would be adding peroxide to a spill whose problem is already peroxide.

A wet vacuum, if the house happens to own one, beats blotting outright, because it pulls liquid up out of the backing instead of asking a towel to draw it. Most households do not own one and should not buy one at nine in the evening for a spill the size of a coaster.

The blotting pattern that stops it spreading

Work the outside of the spill first and move inward. Dye on a landing runs downhill, and landings are rarely level once a house has settled, so check the tread below and the skirting line before you start. A spill contained to twelve centimetres and treated properly reads as a slight shading afterwards. The same spill blotted from the middle outward becomes a forty-centimetre halo with a dark core, and I have taken that job on in Enfield and in Hammersmith and in half a dozen other places, and the halo is always the part that will not come out.

Why does the treated patch sometimes go pale instead of dark?

Because two things were happening at once and only one of them was visible.

While the precursors were building colour on the surface, the peroxide and alkali were working on the fibre underneath, opening the cuticle of a wool yarn and oxidising the dye the mill put there. That damage is invisible while the hair colour sits on top of it. Take the hair colour off successfully with a reducing agent and you expose a patch that has been bleached, usually running orange or pink on a mid-brown carpet, because carpet dye is a mix of components and they do not all bleach at the same rate.

So a successful stain removal can produce a worse-looking floor than the stain did. I tell people this before I start, and a fair number choose to leave the mark alone once they have heard it. That is a conversation that costs me the job about a third of the time, and it is the right conversation to have.

What a reducing agent can and cannot do

The professional route on wool and nylon is a two-part reducing treatment, applied and then driven with heat through a damp towel or with steam. It works on oxidative dye with fair reliability inside the first day or two, works poorly on direct dyes, and does nothing to henna. It also cannot tell the difference between the spill and the carpet, which is the whole problem, and it needs a test in a wardrobe or under the bottom stair before it goes anywhere near the landing.

Timing governs the professional treatment as much as it governs the kitchen towel. Inside twenty-four hours I would expect a good result on oxidative colour on nylon and a fair one on wool. At a week the odds drop sharply, because the dye has been through a full oxidation and the carpet has been walked on, and walking pushes colour deeper into the yarn every time somebody goes to the bathroom.

Polypropylene is the happy exception again. No dye sites, no bonding, and a hair colour spill on a polypropylene twist landing is a detergent job. If your landing is a cheap twist pile in a nineties semi, breathe out.

When is a fitted patch cheaper than another attempt?

Sooner than most of my trade will admit.

A bonded insert takes a fitter forty minutes. They cut a clean square out of the damaged area, cut a matching piece from an offcut or from inside a fitted wardrobe, bond it in on a hot-melt patch tape, and blend the pile direction so the join disappears at normal walking height. In most of London that is a call-out plus an hour, and it is final. Nobody sees it. I have pointed at my own patches in hallways and had the owner unable to find the join.

Set that against two or three reducer sessions, each with a real chance of leaving you a bleached patch that then needs recolouring by someone who does that as a specialism. If the spill is on wool, more than a day old, larger than a beer mat and darker than the carpet, I will usually say patch it. If the spill is small and fresh, I will treat it. If somebody has already attacked it with supermarket bleach or a scrubbing brush, I will look at it, charge for looking, and probably still say patch it.

Where the offcut comes from

The wardrobe floor, the corner behind the bed, the bit under the airing cupboard shelf, the strip in the loft that the fitters left. Every fitted carpet in the country has a donor square hiding somewhere, and finding it is the first thing to do before anyone starts chemistry on the landing. In a house in Wimbledon last year the donor came out of the space behind a built-in wardrobe, and the tenant’s mother had already tried three products from under the sink on a wool carpet that had been beyond them all within the hour.