Most colours belong to no one. International Klein Blue is the rare exception, a shade of ultramarine that carries an artist's name and, in a sense, his claim. In 1960 the French artist Yves Klein registered his particular blue through a Soleau envelope, a French instrument that timestamps an idea without granting a full patent. It was less a lock on the colour than a flag planted in it: this deep, glowing blue, made just so, is mine.

What Klein had actually worked out was not a new pigment but a new way of carrying an old one. The blue was the same lapis-descended ultramarine that painters had prized for centuries, but ordinary binders dulled it, flattening its intensity as they held the powder to the canvas. Klein's real invention was the binder, a medium that suspended the pigment without dimming it, so the colour kept the powdery, saturated depth it has loose in the jar. The secret, in other words, was not the blue itself but the almost invisible glue around it.

There is a long arc in that. The same lapis blue that ruined and enriched Renaissance patrons, that ended more than one workshop's budget, turns up four centuries later no longer scarce at all, cheap enough to buy by the tube, and yet claimed by name as one man's signature. Scarcity had been the story of blue for most of its history; Klein wrote a new final chapter in which the colour is abundant but authored. The rock from Afghanistan had become a brand, and the most famous ultramarine of the modern age is prized not because it is hard to obtain but because someone put his name on exactly that shade of it.