First, Newton saw in the spectrum of only 5 colors: red, yellow, green, blue and purple, but later, yielding to fashion magical figures, decided to increase this number by adding the 2 missing colors. Indeed, the number 7 is the most sacred, revered by many peoples. It is related to fundamental mathematics, microcosm and signifies completeness and wholeness. There are 7 categories of absolute evaluation. In music — the 7 major tones of the musical scale (notes in the scale), a week has 7 days, in the periodic system of chemical elements (the periodic table) 7 periods. This number is dedicated to a lot of Proverbs, sayings and expressions: "try 7 times, 1 time cut", "7 bed — 1 response", "seven of one does not expect", etc.
By the way, not all people believe the rainbow 7-color. For example, some African peoples it 2-color: light-dark. The Yakuts — 3-color: blue-yellow-red, which is reflected on the coat of arms of the city of Lensk. The people of this nation in General constantly confuse colors, denoting cyan, blue, purple and green one word: "cuoh". According to the Chinese, in the 5 rainbow colors and Japanese — 6 (without green).
well, this is all fairly well-known fact that a rainbow consists of 7 colors, red, orange, yellow, green, blue, Indigo and violet. Even there are some verses to make it easier to learn in the correct order all the colors of the rainbow. By the way rainbow is a beautiful phenomenon, a miracle of nature. Appears after rain in Sunny weather, but to see it is not always possible, because it happens in different places. on the Internet there are many photos of the photographed rainbow. It is very like to watch.