Indigo because, let's be honest. Indigo?

But where exactly is violet? Is it at the end here? This dark blue? And what’s this

brighter light blue-green? Cyan, perhaps? Why don't we say the rainbow is Red Orange

Yellow Green Cyan Blue? ROYGCB

Well, we actually do, and we’ve just forgotten. When Isaac Newton originally observed a rainbow

of light split by a prism and made his labeling of the colors as RedOrangeYellowGreenBlueIndigoViolet,

the thing he called “blue” was indeed what we would now call blue-green, teal or

cyan – reminiscent of the color of the blue sky. And what we now tend to call blue, Newton

called violet - as in, roses are red, violets are blue. Dark blue. He only included indigo

in his fundamental "seven colors of the rainbow” so that they would match the number of notes

of the western musical scale: Do re mi fa so la ti… yeah.

Purple and magenta, as we know, don't occur in the rainbow from a prism because they can

only be made as a combination of red and blue light, and those are on opposite sides of

the rainbow – nowhere near overlapping. So there’s no purple or hot pink in the

rainbow from a prism. Violet is there in the “roses are red, violets are BLUE” sense,

but purple is not.

So then why do rainbows in the sky often look like they have purple in them? I suspect sometimes

it's an optical illusion whereby nice deep blues in small amounts surrounded by a lighter

color appear purplish to our eyes. HOWEVER, sometimes purple and pink really ARE there

- because a rainbow is really a rain-disk: each color of sunlight reflects back in a

bright-rimmed disc, all of different sizes, which together add up to make a white disk

with a colorful rim. But because light is a wave, interference from the raindrops themselves

actually gives each disk multiple rings - the familiar outer ring is just the brightest.

The others are called "supernumerary rings" and are the source of supernumerary rainbows

- the smaller the raindrops, the stronger the supernumerary bows. And if the drops are

the right size, the first red supernumerary ring can overlap significantly with the main

dark blue ring, and what do red and blue give? Purple!

So as the saying goes, roses are red, violets are blue, and purple in a rainbow is a supernumerary

hue.