The measured part of a face

Skin, hair and eyes get listed together as though they were three colours of the same kind. Physically they are three different situations, and only one of the three is a hue in any straightforward sense.

Hair is one pigment family in two forms. Eumelanin runs black to brown; pheomelanin runs yellow to reddish. Ito and Wakamatsu's review of the chemistry sets out how the quantities in any real head of hair are known at all: you break each type down to a characteristic product — a pyrrole tricarboxylic acid for eumelanin, aminohydroxyphenylalanine isomers for pheomelanin — and measure the products by liquid chromatography. What comes out is a ratio and an amount. It is not a position on a colour wheel, and the ordinary names for hair colour do not sit on it cleanly.

One hair colour does have a settled cause. Valverde and colleagues reported in 1995 that variants of the MC1R gene appeared in more than 80% of people with red hair, or with fair skin that tans poorly, and in fewer than 20% of people with brown or black hair. That is an association in a sample rather than a test you can sit — but it is why red is the one hair colour with a mechanism attached to it.

Eyes are the odd case. There is no blue pigment in a blue iris and there never was. The only pigment there is melanin, which is brown. Sturm and Larsson's review states the mechanism plainly: where the iris layers hold very little melanin, the collagen fibrils scatter the short wavelengths back towards the surface, so a blue iris is a consequence of structure rather than of a different chemistry. It is the same class of fact as a blue sky. Most of the blue-against-brown variation in Europeans has since been traced to one regulatory site — Eiberg and colleagues mapped it in 2008 to rs12913832, in an intron of HERC2 upstream of the OCA2 promoter, and showed that the variant damps that promoter down.

So: a ratio, a scattering effect, and skin, which is its own subject. What is left that can be measured about a face as a whole? One thing, and it has held up. Porcheron, Mauger and Russell measured facial contrast — the luminance and colour difference between the features and the skin immediately around them — across a large sample of adult women, and found several aspects of it declining with age. Then they raised and lowered those aspects artificially on individual faces. Raised, the same face was judged younger. Lowered, older.

The follow-up is the reason to take it seriously rather than as one result. In 2017 the group measured 763 women in four groups — French, Chinese, Latin American and South African — and found three aspects falling with age in all four: luminance contrast around the eyebrows, red-green contrast around the mouth, blue-yellow contrast around the eyes. Then 108 French and 74 Chinese observers judged 92 faces and picked the higher-contrast version as the younger one in 79.2% of trials, whichever group the face came from.

It is not only age. Russell and colleagues found in 2016 that facial contrast predicted how healthy a face was rated, and that raising it raised the rating. And Jones, Russell and Ward established in 2015 what cosmetics do in these terms: they exaggerate the contrasts that differ between men and women, and the ones that fall away with age. That is the unromantic reason a colour reading is done on a bare face. Makeup is not a distraction from the measurement. It is an intervention on the exact quantity being measured.

Now the part that has to be said. Every one of those studies is about contrast inside the face — a feature against the skin beside it. None of them is about a colour held under the chin, and it would be dishonest to hand them over as though they were. What they establish is smaller, and still worth having: the difference between a feature and its surroundings is a real quantity, it moves with age and with health, observers are demonstrably sensitive to it, and they are sensitive to it in the same direction on four continents. When a personal-colour reading talks about value and contrast, that is the thing it is reaching for. The reading remains a judgement about a face. The axis it is judging on is not invented.