I’ve always agreed with what Alicorn said but didn’t have data. I later heard that it was created by Newton mostly so that the colors could be assigned musical notes, but I don’t know why he wanted to assign musical notes to colors. I also don’t remember the reference.
Since then, however, English has shifted, and this includes color names. The average of the English-speaking xkcd-reading population, which is totally a nonbiased sample, puts indigo at #380282, this dark blue-purple color, but doesn’t really answer it much in comparison to other traditional rainbow colors.
I poked at the Wikipedia article for Indigo for a bit, which unhelpfully describes it as a color between blue and purple. And then I decided that I have no especially strong feelings about colors, if people think it's too purple I will happily agree, if people think it's blue I will also happily agree, it just came to mind as "dark blueish color without blue in the name."
Maggie, IMO white is fine, I want to be more permissive than less on what colors are OK because we've got a lotta glowficcers.
I mean, indigo is a real color, but it is not on par with the six actual rainbow colors any more than, like, "turquoise", which can also be found in a rainbow, is. Indigo is not a basic color word (in English or afaik any other language).
