“Rows of pigments in tubes, jars, and bowls are visible through the doors of floor-to-ceiling cabinets. … There are the products of nineteenth-century chemical innovation — viridian green, cadmium orange, and the chrome yellow with which van Gogh was infatuated but which, over time, has begun to darken his sunflowers. But at the heart of the Forbes Collection are the natural pigments that were the staples of painters’ inventories before chemically synthesized paints replaced the impossibly esoteric, the dangerously toxic, the prohibitively expensive, and the perilously fugitive.”