I've heard about that mental filling of fake colors, I think you're right. I'd like to study it a bit more, though. It just dawned on me writing this! hahaha. The problem I had was I was thinking about color combination mathematically (it can be proved that the combination of two different frequencies gives an envelope wave of the frequency exactly in the middle, so blue and red should make green, not magenta) but thinking about the 3 cones, maybe it's just that one picks the blue part, the other the red part, and since the green one detects nothing, the brain goes haywire and invents magenta. (I think something similar happens with yellow.)
And you're somewhat right about cones. Each type picks up the intensity of light of the frequency they're tuned in, but that frequency is actually a range, and the three ranges overlap with each other. They then send the signals, and then the brain reconstructs the original combination. In our ear, it's actually similar, but we only have one receptor with the whole continuum of frequencies (each frequency of the sound that enters the ear makes the fluid within the cochlea vibrate in a different segment --due to its spiral shape-- and there's "little hairs" that pick up that vibration and send the signal), and that's why I think we have relative pitch with sound, you can compare the different patterns of vibration within the cochlea, f.e. a note will create vibrations in similar sectors than its fifth, whereas with light the signal gets separated in reception and arrives disassembled to the brain, so yeah what you said.
But returning to the Cosmere, I think stuff like the fake colors enters the realm of perceptions, and we're looking for something beyond, or not influenced by, that. It's really hard...
I'm really intrigued about this so I'm happy to be able to discuss it with someone . Sorry if it has veered off too much from the Cosmere.
Edit: Hey @Kureshi Ironclaw, tagging you in case you didn't get a notification without a quote. Sorry if it's a bother you don't need to answer <3.