Customarily, the primary colors of the visible spectrum were red, orange, yellow, green, blue, and violet. However, around 300 years ago, Sir Isaac Newton found another, situated between blue and violet, and named it what?

When Sir Isaac Newton passed sunlight through a prism in the 1660s, he demonstrated that white light was a composite of a full spectrum of colors. Initially, he identified five primary colors, similar to the traditional list. However, Newton was a great believer in a harmonious and orderly universe, and he was deeply influenced by the ancient Greek belief in the mystical significance of the number seven. He saw connections between the seven known celestial bodies of his time, the seven days of the week, and the seven notes of the diatonic musical scale.

To make the colors of the spectrum fit this powerful pattern, Newton decided to divide it into seven distinct bands. He needed one more color to bridge the gap between blue and violet, and he chose to name this new spectral band after a plant-based dye that was being imported from India at the time. This deep, bluish-purple dye was known as indigo, and its inclusion completed his seven-color model.

While many modern scientists and color theorists would argue that indigo is not a fundamental spectral color but rather a tertiary shade, Newton's classification was immensely influential. His choice cemented the seven-color rainbow in our collective consciousness and gave rise to the mnemonic that students still use to remember the order of the spectrum: ROY G. BIV.