Thank god the other Admirals weren’t introduced post-Zou. Imagine a red-haired Akainu, blue-haired Aokiji, yellow-haired Kizaru, and purple-haired Fujitora
Exactly, if we start saying that it was logical for Ryokugyu to have green hair (because it's in his name or in his powers), then we can start saying that it should have been the same for the other admirals.
Blue, red, purple and yellow...
The same goes for Tokikake and Momousaki, who should have had pink and brown hair.
Having said that, I agree with you, it's a real shame that so many people validate this artistic choice, whereas personally I find it really regrettable and inconsistent.
Besides, I don't think Oda remembers, but in the official (canon) color version of chapter 801, Oda's team had colored Ryokugyu with black hair. Proof that before WCI Oda didn't yet have this new contempt for black-haired characters.
https://imgur.io/keBGREt?r