Hermes Greys!

Maybe you can see green under yellow light

If it has a green undertone, it'll show green under a yellow light but even if it has a blue undertone, it'll also show 'green' under a yellow light. That's the light not the colour. There is a spectrum of light absorbed and the yellow bounced back.

You will be able to detect all colours in a flat or almost-flat grey, so depending in which light you place it, you will askew the true colour balance of any grey.

There is no such thing as a neutral grey for all people. Grey is either made from mixing opposite colours together, a secondary + primary opposite (e.g. purple and yellow) or a mix of primary colours (red/blue/yellow). In other words, grey is made-up of all primary colours mixed together, too much red will make it more brownish (taupe) too little will make it look slightly green.

Grey Mayer is not for those that like a brown-grey (even Etain is slightly) or say, Mouette, but then again, the slightly 'cooler' (or bluer) Mouette is not going to be a neutral to others.

Gris Mayer is one of H's more neutral greys, not deliberately a green-grey (like Vert de'Gris) but if people are used to Etain or Mouette, it will not be neutral for them. As you say, in yellow artificial light (or direct sunshine) it will absorb some (more) yellow and that can increase the look of 'green'. The same could be said of many H colours.