The phrase “every color of the rainbow” isn’t quite as all-encompassing as it sounds. For one thing, the color chips in your hardware store’s paint aisle host some colors you’ll be hard-pressed to point to in a real rainbow. But even on a less hair-splitting level, purple is missing from that rainbow.
The V in ROYGBIV stands for violet, sure, but that’s not actually the same thing as purple. There is no purple wavelength of light—it requires a mixture of both red and blue wavelengths. That makes it a “nonspectral color”—in fact, it’s the only nonspectral color humans see. It requires our brains to interpret signals from both red-sensitive and blue-sensitive cones in our eyes and to see that