“A laborious x-ray scan of the painting’s canvas has discovered that Vincent van Gogh used two different types of a chrome yellow paint, one of which is more liable to degrade under light. The change in the 1889 painting, one of a series of sunflowers by Van Gogh, is so far not visible to the human eye but, over time, the painting is set to lose some of its vibrancy in the pale-yellow background and the sunflowers’ bright yellow petals and stems, where the sensitive pigment was mixed to achieve the right green hue.”