Puzzled by indigo - part 1

I've always loved rainbows and had dozens of pictures of them on my wall as a student. But I've never given much thought to their colours. I can chant the Richard Of York Gained Battle In Vain thing to get the red-orange-yellow-green-blue-indigo-violet sequence right, but have always been a bit vague about exactly how blue differs from indigo and how close that is to violet - they all seem a bit too close together compared with the other colours in the spectrum.

Now that I've got hundreds of lovely colours of tapestry wool...


I want to work out which colours make up a rainbow and so thought it was about time I got it sussed.

Turns out I'm not the only one who's troubled by indigo, eg there's a good discussion here about the topic. It looks like I'm going to have to do some more reading to find the RGB/hexadecimal values for my perfect six or seven banded rainbow - and then to translate those into wool colours. Think this minor puzzle is going to turn into a bit of a saga...