Which is another reason it doesn’t make much sense to be in the rainbow, as none of the colors are dark.
The theory I’m more familiar with is that Newton used “indigo” to refer to a wavelength we’d generally call “blue,” and used “blue” for a wavelength we’d generally call “cyan.”
And that how distinct the cyan band appears has a lot to do with whether or not people consider cyan and blue to be two completely different colors. Ultimately, the whole thing is a spectrum: there are no bands of any colors. A rainbow is just a gradient showing all the hues of light that humans can see.