In some repects there is, of course, an infinite number of colours in the rainbow. I heard that when people first thought about the colours, they counted six (red, orange, yellow, blue, violet). However, since seven has long been seen to be a magical or mystical number, it was preferred that there be seven colours, so 'indigo' was invented for the deep blue 'between' blue and violet. They could just have easily chosen 'turquoise' between green and blue or 'vermilion' between red and orange.