It was customary in Old Master depictions of the heavenly host, and of Mary and Christ himself (who is described in the Book of Mark as laden by his tormentors with purple clothes, lampooning his supposed status as ‘King of the Jews’), to transfer the raiments of royal authority to those taking charge of the hereafter. When seen through the lens of the dye’s unsavoury forging, the purple robe that seems forever to be slipping from the suspended physique of Christ in Michelangelo’s dramatic fresco The Last Judgment, which troubles the walls of the Vatican’s Sistine Chapel, can be understood as another tawdry layer of worldly trappings that the Messiah’s Second Coming overcomes. This is the image of a purified humanity coming out of its compromised shell.

That teasing tension between noxious waste and exalted authority, between what’s visceral and virtuous, arguably seeps deep into the weave of every image that relies on the colour for its narrative power. Is it significant that when Raphael, elsewhere in the Vatican, imagines his famous soirée of ancient philosophers, The School of Athens, that his fresco’s central axis should be comprised of a purple-clad Leonardo da Vinci (in the role of Plato) and, just below him, scribbling on a block of marble, a purple-shirted Michelangelo (playing Heraclitus)? That the figures are, albeit in dress-up, the artist’s two most distinguished contemporaries, only amplifies their importance and companionability. Plato (who points upwards) was of course obsessed with all things endless, elevated, and ideal, while Heraclitus was famously forlorn by his focus on the fleetingness of things. Only the conflicted colour of purple – at once exquisite and excretory – could entwine the opposing dispositions into an equilibrium of eternal tension.