Colour lives inside the hair rather than on the surface of it, and it leaves a little at a time with every meeting with water. What keeps colour is several small decisions, each of which takes away slightly less of it than usual.

The first is water temperature. Hot water carries pigment out noticeably faster than warm, and it shows up more in the rinse than in the lather. Cool water at the end of a wash costs nothing and works every time.

The second is what is in the water. Calcium, magnesium, copper and iron from the tap settle on the hair and mute the colour — less that it looks lighter, more that it looks cloudy. Washing once a week with something that lifts those deposits is the practical answer. A shower filter is a weaker one than it sounds: filters are sold against chlorine, not against calcium and magnesium.

The third is the first few days. Fresh colour is still settling, so the first wash is worth putting off for roughly two days. After that, stretch the gaps between washes as far as your hair allows, because each one takes its share.

The fourth is sun. It bleaches pigment out of hair the way a summer bleaches a curtain on the sunny side of a room, and it does the most in the first weeks after a service. A hat works better here than any spray.

The fifth is heat. A straightener or a wand draws colour out along with water, so a heat protectant is working on the shade as well as on the strength of the hair.

The colour will shift anyway. Blonde drifts warm, dark drifts red, pastels fade unevenly. That is the ordinary course of things rather than a failure of care, and a toning product between salon visits handles it better than harder washing.