The colors of the rainbow (a.k.a. Roy G. Biv) have a clear outlier: indigo. Commonly considered a shade of blue, indigo is not a separate color in its own right, so why does it get its own band in the color spectrum?
Indigo was a desired import throughout the 17th and 18th centuries, driving trade wars between European nations and the Americas, fueling the African slave trade, and even partially funding the American Revolutionary War. A natural dye rather than a pigment for painting, indigo was used to color fabrics, clothing, yarns, and luxurious tapestries. Unlike lapis lazuli, whose rarity drove its high prices, the indigo crop could be grown in excess and produced across the world, from India to South Carolina.
Indigo dyeing was especially popular in England, home to physicist Sir Isaac Newton. Newton, who introduced the term “color spectrum,” believed that the rainbow should consist of seven distinct colors to match the seven days of the week, the seven notes in the musical scale, and the seven known planets. Confronting the fact that the rainbow only displayed five unique colors, Newton pushed indigo, along with orange, much to the dismay of some contemporary scientists.
Synthetic indigo, developed in 1880, largely replaced the natural crop by 1913; this is the pigment that dyes your blue jeans. Over the past decade, scientists have introduced a competitor to the market: Escherichia coli bacteria that is custom-engineered to produce the same chemical reaction that makes indigo in plants. This method, called “bio-indigo,” will likely play a big part in the environmentally friendly denim of the future.