When I was a kid we always said "red, orange, yellow, green, blue, purple", but of course there are a lot of transitional shades in between. I'd count indigo as one of those, since it's not a very well-known color by name. In fact, I've never heard of it at all outside the context of rainbows. Nothing is ever described as being indigo in color. If you're going to list indigo, you may as well include spring green, cyan, periwinkle, aqua, teal, fuschia, and chartreuse while we're at it.