Indigo is the color between blue and violet in the spectrum, but almost no one can distinguish that color. Why, then are there seven colors rather than six? It’s because Isaac Newton was drawing an analogy between color and musical tones. There are seven notes in the familiar European scale—A, B, C, D, E, F, and G. So, Newton decided there should be seven colors in the spectrum, and he identified indigo as the seventh. So don’t be too disappointed if you can’t see it.