Indigo is probably the most famous of all natural dyes, and is certainly the most widely used today. What is indigo? The dye is extracted from the leaves of plants in the genus Indigofera, which grow in tropical climates. Dyers made dye by crushing the plant leaves and fermenting them in water. This turns the compound indican, which is a colorless amino acid, into indigotin, a blue dye. The fermented leaves are then mixed with lye, pressed into cakes, dried, and finally powdered.
The notoriety of indigo goes back thousands of years, particularly in India, the oldest center of indigo dyeing. India was the primary source of indigo in the Greco-Roman era, though other Asian countries like China and Japan had also utilized indigo for centuries.
It became an incredibly valuable resource in Europe through the Middle Ages, becoming only slightly less rare as trade routes opened up throughout the Renaissance. Demand for the dye fueled trade wars, propelled the slave trade, and partially financed the American Revolutionary War.
Indigo’s Impact on Science
Indigo fever even had an impact on science and physics that lasts to this day! In the mid 17th century, Sir Isaac Newton was proving that white light comprised a full spectrum of colors. He demonstrated that light could be divided by use of a prism, then reunited with another prism.
He divided the indistinct continuum of color into seven distinct, visible colors: red, orange, yellow, green, blue, indigo, and violet. So what is indigo doing in the color wheel? Newton’s distinctions and color names were somewhat arbitrary, as is the nature of color in general. Indigo was a major commodity at the time, and it would have been recognizable to Newton’s contemporaries.
The way you see color depends upon the context, your culture, and the language you have to talk about color. The only reason we see a rainbow made up of bands of color at all is because human perception prefers to organize stimuli into distinct “buckets” rather than dealing with a spectrum. It’s a shortcut your brain takes so it doesn’t have to work as hard.
Newton decided to divide the rainbow into seven colors because he believed seven was a cosmically significant, even “magic” number. The musical scale has seven notes, and Newton decided to define seven distinct colors as well.
Modern Color Wheel Controversy: What Is Indigo Doing In There?
Today, many color specialists advocate for removing indigo from the colors of the rainbow. They want to define it as: red, orange, yellow, green, blue, purple. Today, the color “indigo” refers to a blue-purple color that many people can’t actually distinguish from blue or purple. Experts say that Newton only put indigo in the rainbow because he wanted seven colors, and indigo was an extremely valuable commodity at the time.
But many believe that what Newton called “blue” was closer to the modern “aqua,” a mix between blue and green, and that his “indigo” was what we’d call “blue” today. This makes sense if you consider the actual color of indigo dye cakes. They are really not very purple at all. In which case, it’s not his decision to define seven colors that’s the problem—it’s the naming conventions that are the issue.
In today’s world where most blue dye is synthetic, calling blue “indigo” doesn’t make a lot of sense. It’s especially difficult to teach to children, who may be wondering, “What is indigo?” It might make more sense to change the rainbow to a more modern version, still with seven colors: red, orange, yellow, green, aqua, blue, purple!