The traditional description of the colors of the rainbow (since Isaac Newton established it) is that there are seven of them: red, orange, yellow, green, blue, indigo and violet. “Indigo” is one that has kind of annoyed me since childhood: the other six color descriptors are all hues (i.e. base colors), whereas indigo is a nuance, thus only a shade of a color. By that standard there should not be seven, but six “main” colors in the rainbow; otherwise, the “blue” stripe would be better described as “turquoise”, “teal” or “cyan” (Newton, who has been described as the last alchemist and the first modern scientist, assigned 7 colors to the rainbow because 7 is a “magic number” and corresponds, for example, to the 7 notes of the major scale; read: unscientifically). But let’s talk a bit more about that contentuous stripe. When I hear “indigo”, I understand that to mean a very dark unequivocal blue. It’s a natural pigment from which blue dye was traditionally made and I take that shade to mean the color of a clear night sky, or of dark wash jeans. Yet in childhood I somehow got the impression that it was more of a violet. This impression may have been created by a realistic painting of a rainbow that I saw that I mistook for a photograph. I even once debated my second-grade teacher on the matter! But looking at a rainbow with the naked eye, the stripes are too thin to make the exact color out clearly.
In actual fact, is there any substantial red content in the “indigo” stripe, which would make it a shade of violet/purple (or at least “blue-violet”, I.E. the tertiary color), or is it really a more or less pure blue? Has this even been determined scientifically, or is it purely a matter of perception? Is it more of a gradient? I.E., is the “indigo” stripe actually part blue and part violet?