Thirteen beliefs colourists hear every week, from “higher volume covers grey better” to “purple shampoo fixes brass for good”, checked against regulations, manufacturers’ instructions and published studies.

Colourists treat porous ends as quick to grab. It is a sound working rule, but no study has measured it for permanent colour, and the reason to strand test is the reason the evidence is missing.

Permanent colour fades. Light and washing are the measured causes, and they do the most damage together. Colour on lightened and white hair shows the most change. Here is what has been measured and what hasn't, and what to tell clients at consultation.

"Ammonia-free" covers two different products. Ammonia-free demi is designed to last 12 to 24 washes. For ammonia-free permanent, the only statement on record is Henkel's own patent, which rates MEA-only systems poorer for wash-fastness. This guide covers what replaces ammonia, what the damage studies measured, and what to read on the label.

No: keep each bowl to one brand. Schwarzkopf, Wella, L'Oreal and Matrix each write their shades, developer and ratio for their own range only, so a two-brand bowl is off-instruction for both tubes. Colouring over another brand's previous colour is a different question. Change brands between services, not inside one bowl.

On paper, equal parts of 20 and 40 volume developer give about 9%, the strength sold as 30 volume. That sum assumes both bottles are true to label and measured accurately, and we found no test showing a blend behaves like a factory bottle. If the formula calls for 30 vol, use a 30 vol bottle.

PTD is chemically PPD plus one methyl group — which is exactly why the ASA upheld a complaint against calling it 'PPD-free'.

Different alkaliser, different peroxide range, different timing — and no published test of what happens when you combine them in one bowl.