Why doesn’t magenta appear in the rainbow or in a spectrum cast by a prism?
Video Link) This minute-long video explains that magenta (or "pink" as they call it here) is an "extraspectral" color. It's an invented color made to fill the gap between red and blue.
One way to achieve magenta is to overlap the two extreme ends of the spectrum.
Magenta is considered a primary color of printing ink, developed in the 1890s. It's the "M" is CMYK. For the purposes of charting the color universe, we regard it as a pure color, but it’s really a composite of red and blue.
Magenta sits directly opposite green on the "Yurmby" color wheel. You can get magenta light by subtracting lime-green light from white light. Magenta is also an afterimage of green. Look at that green dot for 30 seconds and then look at white screen to the right, and you'll see magenta. This is a reminder that color doesn't really have an objective existence apart from our perception of it.
Color theory is full of these niggly exceptions, and that's why it's so challenging to write about. Color theory just doesn't come out neatly like a geometrical theorem.
In the artist's practice, it lives at the intersection of the vagaries of visual perception, the chemistry of pigments, and the physics of optics. I was aware of all this when I wrote Color and Light, but if you include all the qualifications and footnotes, the book would have been 1000 pages instead of 224. I see the blog as a way to extend the book. And maybe that's something an app edition could do.
Magenta on Wikipedia
Check out the "Rotating Dot Illusion" from Biotele