Seven is a common story-telling number. Seven days in a week, sailing the seven seas, the seven wonders of the world, the seven continents, seven deadly sins, etc.
Seven colors. There is a whole spectrum of color, but there are only a handful of colors that most humans distinguish and talk about. It has always seemed odd to me that we elevate Indigo into the spectrum of named colors. However, Newton was fond of the occult, and numerology, and the number seven. He wrote papers about appearances of the number seven in the Bible. I think that Newton made Indigo an official color j…
Note that different cultures do not actually agree on 7 colors in the spectrum or which colors they are. For example some languages consider green to be a shade of blue or vice versa, or green to be a shade of yellow. Some only distinguish between hot (red-yellow) and cold (green-blue) colors. There are also some non-spectrum basic colors, for example english considers light-red (pink) and dark-orange (brown) to be basic colors, whereas a dark green is just a shade of green; and this also varies by culture.