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Sir Isaac Newton, great scientist that he was, had an occult side that has left us a flaky definition of which colours are in the rainbow. But today popular culture is changing his colours.

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Newton was a pioneer in research on light. He loved to put plain sunlight through prisms and watch it change colour.

And he set out the colours of the rainbow in an order schoolchildren still memorize today: red, orange, yellow, green, blue, indigo, violet.

That’s seven colours. Three centuries after Newton, you can still read about them anywhere.

Then a funny thing happened. Modern portrayals of the rainbow have reduced the number of colours to six.

The multicoloured banner seen at Gay Pride events is a rainbow of sorts. But if you look closely, you’ll see it’s missing a Newtonian colour. Indigo has vanished, leaving three primary and three secondary colours.

The Gay Pride flag eschews indigo in its representation of the rainbow. Just an accident, perhaps. Someone forgot to check a physics textbook before sewing a flag, which isn’t exactly a shocker.