The Emperor Augustus boasted: "I found Rome a city of bricks and left it a city of marble." And that, for most visitors, means a city devoid of colour: gleaming marble statues, pristine columns and white monuments. Their exteriors have been washed clean by time and weather, their interiors neutralised by the wear of thousands of years.
An exhibition that opened last week at the Scuderie del Quirinale - a museum of classical Roman art up to the 19th century - reminds us that the city was once awash with colour. Rome: The Painting of an Empire shows Roman life for the first time in a dazzling assembly of frescoes, decorations, painted panels and murals from the first century BC to the fourth century AD. The museum's scientific adviser, Caterina Cardona, says: "We have always had the idea that the past was very white. We think that the statues were white, but in Roman days there was plenty of colour which was integral to every house, every column, every statue."
The exhibition also places the works in a context that illustrates how the paintings can serve as a bridge between the Greeks and the European impressionists many centuries later. "The picture was born in Greece, where they were used to great art," Cardona says. "But there were no artists here, just artisans - almost like painters and decorators. We know nothing about them but the exhibition shows that the pictures of Rome have their own dignity and their own significance and that, in a way, the true art of Rome was not the sculpture but the painting.
"That is why we have presented it not as an archaeological exhibition but as one of paintings." The curator Eugenio da Rocca says: "We are celebrating 10 years as a gallery and it feels right to show an exhibition of figurative, classical art like this. Our aim here is to go deep into Italian culture." The Scuderie also plays a part in Rome's story. It was once the stables for the popes, kings and presidents who lived behind the inscrutable 18th-century palazzo on the opposite side of the Piazza del Quirinale Then it was used as a garage, complete with a petrol pump on the first floor, and briefly, a museum for carriages.
Today, at the top of the long, shallow, sloping staircase where horses once hauled carriages, darkened galleries reveal a collection of dramatically lit paintings. They tell a story of Roman life that is charming and surprisingly modern. Many of the works would have been painted into niches or on shelves, doorways and walls. Many tell mythological stories, but the ones that catch the eye reflect the fact that wealthy Romans were every bit as materialistic as their 21st-century heirs, decorating their walls with art and their homes with objects, not to mention themselves with fine jewellery.
Da Rocca says: "The life in the Roman villas was like a dream. The rich Romans had paintings on their walls and, as you see here, because they did not use perspective, the images of landscapes and characters seem to flow into the air. From what we normally see, you cannot guess there was so much colour." The traditional view of Roman artists is that they copied the Greeks, though it is probable they were also influenced by Etruscan and Egyptian artists, using tempera and gouaches as their basic materials. One of the best examples of the way they worked without a sense of space or perspective is Landscape With Small Boat on a Black Background from the National Archaeological Museum in Naples. It shows people and villas on a sea shore who bear little relationship with the boat at the foot of the picture. Its one-dimensional character - like many others - creates the dreamlike effect that da Rocca talks about.
The exhibition's works come from London's British Museum, Edinburgh, Zurich and the National Archaeological Museum's collection of artefacts and murals from Pompeii and Herculaneum, which were destroyed by a volcano in AD79. They capture a world of myths, gods and goddesses at leisure, couples embracing, musicians playing, food and drink being enjoyed. The tempting come hither of the woman on a bed, captured on a fragment from Pompeii, is sweetly sensual while the fierce, ill-fated, embrace of Polyphemus and Galatea captures the desperation of a love that would end in death. To emphasise the colour of the original, a reconstruction of the entire panel, the frame and the background that originally contained the picture is a bold mix of reds, blacks, blues and golds.
There is a frieze of maenads (the female followers of Dionysius) skipping and dancing; naughty cherubs make music and fish; a nervous bride-to-be is reassured by her attendants. There are familiar works, too, such as The Three Graces from Pompeii and the Head of Medusa from Herculaneum. Above all, it is the portrayal of the humdrummery of Roman life that gives the exhibition its appeal. The small internal world of the otium, the reception room where Romans would relax after a busy day at the senate, was the space for lazing around and meeting friends. A series of still lifes capture this world, with panels showing what would have been in the villa: grapes, eggs, bread and coins (the viewer is reminded that the Romans of 2009 still prefer cash to credit cards). A banquet scene contrasts with the distribution of bread for the poor. A lobster, eel and squid do battle in a surreal fragment from a Roman house.
Many of these endearingly everyday objects were influenced by the Greek artist Pierrakos, who, in the fourth and third centuries, was known as the painter of "contemptible things" such as barber shops, donkeys and cooked food. Pressed to name the most important piece - "We chose the best, so I can't say" - da Rocca admits that his favourites might be two large murals taken from the Villa della Farnesina, a late Republican mansion. One is full of light and bucolic scenes of country life and sea, the other is a dramatic black with faded depictions of horses, sanctuaries, statues, humans and animals - the daily business of work, prayer and pleasure.
"Again, it is as if the gods are dreaming," he says. "You have to imagine that the black background would have been in a deep, shining, gloss. It is as if a rich Roman was interpreting the life of the gods. Just imagine this 2,000 years ago. Black walls had never been used before and this one contrasts with the lighter mural that would have been further down the corridor. It launched a new fashion, with the characters and scenes picked out in gold leaf and white, glittering in the light of the candles."
There is nothing dreamlike about the section of portraits. Strong faces stare back at the visitor with a realistic intensity. Apart from a few found in the cities near Vesuvius, the finest collection of portraits is from the fourth century and the Egyptian oasis of El Fayoum. "These were funerary portraits, not house portraits," says da Rocca. "The face was put onto the mummy with paintings using encausto - melted wax mixed with colour and applied like a palate. They would use lime wood and sometimes linen.
"The artists were very professional and came from Alexandria, which was a very big centre for art and artists in general. We don't have many other portraits from ancient times because the idea to put the portrait on the tombs was unique to the Egyptians and to this area of El Fayoum. "At the time, Egypt was a mix of Greeks, Egyptians and Romans, but the culture was Roman - everything about the way they are represented is Roman, their dress, their hair.
"The portraits are very modern. We can see that many of the painting techniques come from Greece but if you look at one of them, the woman who we have borrowed from Edinburgh, you can see how similar it is to portraits by the impressionists. It's very interesting. I'm not saying it is like the impressionist movement of Fauvism with its use of dots, but it is very near. You see the colour, the detail and the relief, and you can see that this work is looking ahead to the art of the 1500s, even the 1800s. It is a bridge between the Greeks and the impressionists.
The woman he refers to is a striking figure with fantastically tight-curled ringlets, huge earrings and necklaces known as The Golden Girl of the Jewels. With her arched eyebrows, she cuts an imperious figure like some Medici princess or a grand dame of the theatre. "She is very elegant," da Rocca says admiringly. "She is a very Roman lady but could almost be Parisian." Rome: The Painting of an Empire is at Scuderie del Quirinale, until January 17. Visit www.scuderiequirinale.it for more information.