In 1892 Claude Monet had a nightmare in which Rouen Cathedral fell on top of him. It would seem an ironic fate for a painter of evanescent effects to be buried under a pile of masonry, but he was wrestling at the time with a major series of paintings of the massive structure, sometimes working on up to 10 at once. The great Impressionist naturally dreamt in colour. In his dream, he saw the stone as “blue, pink or yellow”.