The spectrum in the image below was created from sunlight diffracting through a crystal ornament, hanging in my living room window.

The visible spectrum contains all of the primary and secondary additive and subtractive colors except one. Can you tell which one, just by looking?

It may be hard to see from the picture, but all of the above colors are present in the visible spectrum (the rainbow) except for magenta. Additively, magenta is created by an equal mixture of red light and blue light. Because red and blue are on opposite sides (literally) of the visible spectrum, they are never side by side, and thus their mixture never arises. All other secondaries in the above table lie at the intersection of two primaries (orange is between yellow and red, cyan is between green and blue, etc). Magenta will never be seen in the spectrum because red and blue cannot be side by side.

Unless…

Two spectra, just by coincidence, overlap.

Two overlapping spectra, creating magenta at the intersection of red and blue.

A rare event? Interestingly, I noticed this phenomenon for the first time, on my living room ceiling, the day we discussed magenta’s absence from Newton’s (ROY G BIV) color scheme during our last meeting.