Two clients ask for the same shade, and you mix the same tube with the same developer for both. One comes out exactly as planned, the other turns muddy, ashy, or fades back to brass by the third wash, and nine times out of ten the difference is porosity.

Porosity is how readily hair takes water and colour in, and how well it holds onto them once they're there. Raised, damaged cuticles drink pigment up and let it wash straight back out, while tight, glassy cuticles resist it going in at all. The colour chart assumes even, undamaged hair, and almost nobody who sits in your chair has hair like that.

Reading porosity before you mix

The client's colour history tells you most of what you need before you've touched the hair. Box dye, home bleach, a bond-broken balayage from another salon, straighteners run at 230 degrees every morning, regular swimming, years of lightening through the same mid-lengths: anything that's lifted, heated, or chemically treated the hair has raised the cuticle and pushed porosity up.

Then feel the hair by running a damp strand between your fingers from root to tip. A smooth, slick strand that squeaks usually means low porosity and a cuticle that will resist you, while a rough, spongy drag, especially through the ends, means the hair is porous and primed to grab. Most heads are a mix: resistant virgin regrowth at the root, thirsty over-processed ends, and a band of unevenness in between that's exactly where colour goes patchy.

That gradient matters more than a single label. The root, the mid-lengths and the ends can each behave like a different head of hair, which is why a single formula painted wall to wall lands three different ways on the same client.

Porous hair

Porous hair grabs pigment fast and pulls it cool and dark. Go for a warm caramel and the ends drink up the ash base and come out flat and greyish. Take someone darker over previously lightened ends and the porosity swallows the colour, often flashing green or muddy because there's no warmth left underneath to balance the deposit.

Porous hair lets go just as fast as it grabs. A vibrant copper or a fashion shade on bleached, porous ends can look perfect at the backwash and be half gone within a fortnight, because the cuticle that let the pigment in can't hold onto it. Clients usually blame the shade, and the real fault is the condition of the hair.

The fix is to even the hair out before you deposit. Pre-fill porous ends with a warm shade a level or two down from your target before going darker, so you're replacing the missing red and gold the colour needs to sit on. A protein-based filler or a porosity equaliser does the same job and stops that patchy, blotchy take through over-processed sections. Drop your developer too: 10 or 20 volume is usually plenty on porous hair, because the cuticle is already wide open and a higher volume blows the colour through and out the other side.

Resistant hair

The opposite problem is the glassy, low-porosity head that won't let anything in. Coarse virgin hair and stubborn greys are the usual culprits, and when grey coverage fails, the processing is usually at fault rather than the product.

Resistant hair needs the cuticle coaxed open and time to work. Pre-softening tough greys, leaning on a higher volume where the lift calls for it, and giving the colour its full processing time rather than whipping it off early all make the difference between full coverage and a halo of sparkly grey around the hairline by week two. Heat helps, used deliberately, and never on hair that's already porous.

Questions to ask at the consultation

You can save yourself the surprise by asking before the trolley comes out, because the answers often tell you more than the hair will admit on its own:

  • What have you put on it yourself? Box dye and home toners build up and skew porosity unevenly, and they're exactly the products clients forget to mention.
  • How quickly does your colour fade? A fast fade points to porous hair, while stubborn grey that grows out fast points to resistance.
  • What heat do you use, and how often? Daily straighteners and hot tongs raise porosity through the ends regardless of what's been on the hair chemically.
  • Do you swim? Chlorine and salt are quietly brutal, and a regular swimmer's ends behave nothing like the regrowth.

None of this replaces a strand test on hair you're unsure about, especially before a correction. Five minutes on a cutting from the most porous section will tell you whether the formula holds or pulls before you commit to the whole head.

Keep a record for the next visit

The most expensive mistake is solving porosity once and then forgetting what you did. The client comes back in six weeks, a different stylist takes the appointment, and the whole puzzle gets worked out again from scratch, sometimes with a worse result.

Keep the detail on the client's record: how the hair behaved, what you filled or softened, the developer you dropped to, and how the colour wore over the following weeks. Then the second visit starts where the first one finished, and you reach straight for the formula that worked.

Porosity is the variable behind most colour that comes out wrong. Learn to read it at the consultation and adjust the developer and the deposit to match, and the same tube behaves the same way on every head.