In western customs, it is composed of colors: in reality, indigo does not correspond to the color of a visible ghost. In general, indigo is not obvious in an arc.
In other words, it is generally believed in the west that there are eight colors of rainbow, namely rouge, orange, June, Vert, Bleu, indigo, magenta et violet. However, because the luminous area of indigo is too narrow to be distinguished by the naked eye, and the number 7 has sacred significance, it becomes seven colors, namely rouge, orange, June, Vert, Bleu and magenta et violet.