Yellow-green shade can be a single, monochromatic spectral spike between green and yellow wavelengths (what you describe), or it can be a mix of pure green and pure yellow wavelenghts, or it can be a mix of pure green and pure red wavelengths, the red one being much lower in level, or pure green and orange, or it can be a continuous region from green to yellow, or from green to red, lowering in level towards red, etc. etc. etc. If you give up a little in saturation (which is hard to evaluate, after all), you may even have a mixture of bluish-green and pure red, and that mixture still looks like yellow-green!
Originally Posted by JPD
So again, there are countless possibilities and our eyes cannot make difference. Some of them may be rendered differently by different color films or different digital cameras, but mostly they try to mimic the eye vision as much as possible.
Our eyes only detect three broad overlapping wavelength regions. So, monochromatic, pure yellow gives equal signal to both "red" and "green" detectors because the spectral sensitivities of "red" and "green" detectors in our eyes overlap. If they didn't overlap, we would see monochromatic "in-between" colors black which wouldn't be very nice. And, for example, a signal of 80% "green" 20% "red" would translate as "yellow-green shade" in our brain.
The only thing we can judge by our eyes to some extent, is that when we see VERY pure and saturated RED, GREEN or BLUE, then we can know they are relatively narrow spikes relatively near to specific wavelengths, and there are "not very much" other wavelengths present. But recognizing these very pure and saturated colors is not that trivial, and this works only for just those three colors.