my reasoning: the actual colors we can see -> the wavelengths that we can extrapolate to -> basically extrapolated wavelengths plus an 'unpure-ness' factor -> not even real wavelengths (ok well king blue and maybe lavender if I'm being generous could be)
That is what the brain does and it does it all the time with everything. You can have a simple B/W picture and you/your brain will see motion.
That's not true though. Yellow is a very real wavelength. The fact that we don't have unique receptors for it changes nothing about it's nature. Also our eyes are not being tricked into anything, they just pass on the signal they get. Our brain sometimes plays tricks on itself. But interpreting red and green signal as yellow means getting closer to the physical truth, not further from it.
All receptors are always triggered for all wavelengths we can see, just some more than others. Different color have different ratios. An exactly 540 nm green light would still trigger all 3. It is all your brain doing the processing and those 3 primary colors of our usually 3 cone cells are the opposite of spectrally pure. Otherwise you would not even be able to see those mixed colors.