White light scatters into a spectrum of colours that blend into each other- there aren’t discrete lines. The most obvious colours we have well-known names for are red, orange, yellow, green blue and purple- six. However since seven is a “magic” number, people insist on splitting one colour into two shades. In the west, especially since Newton, we “see” two shades of blue, blue and indigo, but in China seven-colour rainbows are drawn with two shades of green and only one of blue.