From the Times:

We are so used to the white purity of ancient marble sculptures that we imagine the Greeks and Romans felt the same: certainly the artists and patrons of the Renaissance and later centuries believed that white was right. New research using strong raking light sources and beams of ultraviolet light has shown, however, that many Classical statues were gaudily painted in a plethora of colours.

“The ideal of unpainted sculpture took shape in Renaissance Rome, inspired by finds and early collections of Classical marble statues such as the Laocoön, discovered in 1506, said Dr Susanne Ebbinghaus of Harvard University, organiser of a recent conference on Gods in Colour. These were denuded of their painted surfaces by prolonged exposure to the elements, burial and often, most likely, a good scrub upon recovery.”

Michelangelo famously rated sculpture much higher than painting, and Vasari ignored polychrome decoration except on wood carvings, and the impact of statues such as Michelangelo’s David established white marble sculpture as the noblest of the arts, something that continued from the Renaissance into the neoClassical period of the 18th and 19th centuries and the establishment of an art-historical canon by Johann Joachim Winckelmann.

The idea of plain white marble seems to be earlier, however, if the figures of the Virtues and Vices in Giotto’s Scrovegni Chapel in Padua are anything to go by: the Renaissance ideal existed, at least in painted portrayals of sculptures, from around 1300 in Assisi. Colour was indicated by texture in marble carvings, the smoothness of flesh allowing the inner tone of the stone to show through, while various roughenings suggested fabric and leather.

When ancient sculptures began to be unearthed early in the 19th century, such as those from the Temple of Aphaia on Aegina, excavated in 1811, significant traces of paint were visible. Reconstructions on paper of presumed original colour schemes engendered debate as to whether white marble was still desirable, and by midcentury John Gibson had created the Tinted Venus in emulation of Praxiteles.

However, “we have still not come to terms with the painted marble sculpture of Ancient Greece and Rome,” Dr Ebbinghaus said. A campaign of research led by Dr Vinzenz Brinkmann, of the Liebighaus museum in Frankfurt, has now tried to tackle the problem.

One of his main tools has been the use of strong raking light, which can show finely drawn incised sketches to guide the painter: a lion’s head on the shoulder guard of the famous Stele of Aristion in Athens is one example, and several Cycladic figurines from the Bronze Age have similar sketches two millennia earlier. The use of ultraviolet light can show up the “ghosts” of former painted areas.

Brinkmann and his wife, Ulrike Koch-Brinkmann, have produced copies of a number of Classical sculptures where such evidence is apparent, using natural mineral and plant pigments available to the ancient artists and identified by X-ray fluorescence, infra-red spectroscopy and other high-tech methods. The Aegina pediment sculptures used copper-based pigments such as azurite, Egyptian blue and malachite for blues and greens, cinnabar and ochre for reds, and also the plant extract madder.

A figure such as the Trojan Archer on the pediment explodes into a riot of colour, including polychrome trousers where the design stretches with the movement of the limbs and a bright yellow jerkin and cap. The Aristion stele uses cinnabar, madder, malachite, ochre and Egyptian blue, and the famous “Alexander Sarcophagus” in the Archaeological Museum, Istanbul, all of these, together with minium (red lead) and sienna.

The Renaissance idealisation of monochrome sculpture would have startled the Ancients, Ebbinghaus said. “Just as the colour reconstructions of ancient statues startle us today. It is difficult to imagine a fully coloured sculpture, complete with additions in other materials such as metal, or eyes inlaid with glass.”

Folks should take Mary Beard's comments to heart on this one ... I've been thinking that there might also be a difference between sculpture designed to be outdoors (which would be exposed to the sun and be in need of some protection perhaps; over time the colours would be muted) and indoors (which might not have been painted).