"Link" to what "source"?. My gosh, is this an era where alleged research is just a click of a mouse away form some half-baked web opinion somewhere or other? Guess so. But I certainly wasn't implying that you or anyone else here has a Nazi mentality. I've actually seen Nazi eugenics manuals at rare book shows, and how they tried to lump various human features into very narrow categories and stereotypes irrespective of any real genetic science. And that's the only kind of place I can think of where a pseudo-study would have been made about the tone of people's palms. And like I said, the whole notion was sound ridiculous to a dermatologist or ethnologist who has seen many many hand. I was just employing that as hyperbole. There's nothing that sinister in any of this palm reflectance value talk.
But if you want to make an analogy between skintone and gray cards, I've never seen a living human being that looked gray other than gray hair. Palms might be whitish, or pinkish, or all kinds of flavors of tan and brown, while all my light meters all have peak sensitivity at green, just like most modern meters and human vision itself. Neutral Gray reflects green and other colors neutrally, while pink or tan or whatever do not. One more strike to that idea of realistic interchangeability.
And the presence of color on your "standard" does make a significant difference. That's why a serious color chart like the MacBeth includes representative skintone patches as well as neutral gray and strongly colored ones. An 18% tan is going to give a different reading with a common light meter than the neutral gray 18% version. They base it on spectrometer reading. I've spent many many hours studying all this in detail in my own lab, with my own special instrumentation. I'm not against the casual use of palms when the nature of the film itself forgives it; but being from a less forgiving chrome film background, I sure know the difference, and the pitfalls involved.