answers: In the mid-1660s
context: human urine, ideally urine from men who had been drinking a great deal of alcohol, which was said to improve the colour. The fabric was then soaked for a day in the resulting mixture, then put out in the sun, where as it dried it turned blue. The pastel industry was threatened in the 15th century by the arrival from India of the same dye (indigo), obtained from a shrub widely grown in Asia. The Asian indigo dye precursors is more readily obtained. In 1498, Vasco de Gama opened a trade route to import indigo from India to Europe. In </s> introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of prisms at a fair near Cambridge, the East India Company had begun importing indigo dye into England, supplanting the homegrown woad as source of blue dye. In a pivotal experiment in the history of optics, the young Newton shone a narrow beam of sunlight through a prism to produce a rainbow-like band of colors on the wall. In describing this optical spectrum, Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary </s> dark blue color. Literature Marina Warner's novel "Indigo" (1992) is a retelling of Shakespeare's "The Tempest" and features the production of indigo dye by Sycorax. The French Army adopted dark blue indigo at the time of the French Revolution, as a replacement for the white uniforms previously worn by the Royal infantry regiments. In 1806, Napoleon decided to restore the white coats because of shortages of indigo dye imposed by the British continental blockade. However, the greater practicability of the blue color led to its retention, and indigo remained the dominant color of French military coats until 1914. The spiritualist
question: when did indigo become a color of the rainbow