fake is just an easy way to communicate the idea without going into a bunch of complex color terminology. extra-spectral is a name for them if you really want to split hairs about it. which includes both impossible and imaginary (which are also described as fictitious).

I feel it's like saying "matter is mostly empty space and objects don't actually touch, they repel each other". Yes, that may be true on a physics/atomic level, but on a practical, every day level, objets are "solid" and they "touch".

Yeah, pink/brown doesn't "exist". There is no "pink wavelength". It's "a composite". But you can still pull a pink crayon out and everyone agrees "yeah, that's pink".

Saying colors don't exist is splitting hairs in a context most people aren't referring to.

In the case of the shrimp, it does matter because are they seeing "pink" or "red while also seeing purple separately and distinctly"? It's asking if they are processing the colors in the same format.