In the regular list of the rainbow, the color indigo does not belong. Don't go listing the rainbow like "Red, orange, yellow, green, blue, INDIGO, violet!" Indigo is just the combination of blue and violet. The whole rest of the rainbow doesn't include the tertiary colors (colors that are a mix of two secondary colors, such as red and orange making red-orange; orange is a secondary color, red is a primary, and red-orange is a tertiary), so why would you include indigo? What makes indigo special? Indigo is just another name for blue-violet. You could argue that "indigo" is easier to say than other tertiary colors like yellow-green, but what about cyan? Cyan is nothing but blue-green, and it's not included in the typical rainbow list, so why is indigo? It shouldn't be! That's the answer.
Indigo is a beautiful color but it does not belong if you are simply listing off the basic colors of the rainbow. This concludes my TED Talk. Thank you.