By using superprecise lasers to activate naturally impossible patterns of light-sensing cells, researchers showed five people an intense blue-green, which has never been seen before by human eyes. They call it “olo” (from zero-one-zero, its colour triangle co-ordinates).
Olo: a colour never yet seen
Whitby Writers Group 
“It was octarine, the colour of magic. It was alive and glowing and vibrant and it was the undisputed pigment of the imagination, because wherever it appeared it was a sign that mere matter was a servant of the powers of the magical mind. It was enchantment itself. But Rincewind always thought it looked a sort of greenish-purple.” – Terry Pratchett, The Colour of Magic
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On reflection, greenish-purple is subtly opportune. I can think of two others to go with it: blueish-yellow and reddish-turquoise.
All three should look much the same: a sort of off-white.
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