As much as I admire your jumper, said Pliny thoughtfully, I prefer the colour indigo, which is a darker shade of blue. We ancients valued it highly. We imported it from India, in the form of a blue pigment, and used it extensively in painting and dyeing.
I like indigo as well, I said. But nowadays it's considered by most modern colour scientists not to be a separate colour at all.
Surely not! said Pliny, looking shocked.
Oh yes. Indigo has a wavelength between 420 and 450 nanometres, and anything with a wavelength shorter than 450 nanometres is classified as violet.
Outrageous! It looks nothing at all like violet.
That is probably what Newton thought. Although, apparently when he first identified the colours in the optical spectrum he only differentiated five. It was only later that he added orange, and then indigo.
After he had looked a little more carefully, nodded Pliny.
No, after he had decided that there ought to be seven colours in the spectrum, to match the number of days of the week, the number of notes in the major scale, and the number of planets that were known at the time.
Aha, said Pliny. I see. Now that makes perfect sense. Who is this excellent Newton?
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