Violet is a real wavelength, below blue on the spectrum. Where it becomes invisible to the human eye, it starts getting called ultraviolet.
Magenta and purples are constructs by the brain, as you mention.
Magenta and purples are constructs by the brain, as you mention.
If I shine some wavelength to your eyeball and you say "it looks blue", but then I change the surrounding and now it looks white, I don't think you would conclude that the original wavelength is blue.
We have a many examples like this, which prescribe that vision is not at all an accurate wavelength measurement device.