In my opinion, either that or an Eastern mythical creature that relates to time directly or indirectly (such as symbolizing time and longevity; for example, through vegetation). Time not only fits the physics trio but is also depicted as green in tons of popular works.
What is clear here is that every admiral's colour theme was linked to their ability, from Kuzan to Fujitora. Odds are Ryokugyu has a "green ability", and Minotaur wasn't green.
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Not exactly. The literal translation of Marco's fruit is "Bird Bird fruit, model: immortal bird" (トリトリの実 モデル"不死鳥"). In other words, as it is written, his power is "Tori Tori no mi, moderu: Fushichou". "Phoenix" (Fenikkusu) appears in its furigana (フェニックス), that's indeed true, but Fenghuang is an "immortal bird" too. In my opinion there's some intended ambiguity there that fits what I explained before and in my next paragraphs.
It's not as simple as you are making it seem because, as I said, modern culture pretty much merged both animals and differentiates them as "Western phoenix" and "Eastern phoenix". In current times, the interchangeability of both creatures is common; for example, the Phoenix constellation, which is named after the Western version, is actually called "Fenghuang constellation" in Chinese and "Houou constellation" in Japanese.
When you argue that Marco's fruit is the Western phoenix because that's its name, it's just a very shallow understanding of this issue, as I've been telling you (with no mean intention). The modern world basically sees the Fenghuang/Houou as a phoenix. Pop culture gave it the powers of the Western bird (again: Ho-Oh and Hououmon are the best examples of this) and "(Greek) phoenix" has been translated to "Fenghuang" and "Houou" tons of times (and vice versa; since I already mentioned it, Hououmon is named Phoenixmon in the West).
Considering all of this plus how many Fenghuang traits Marco has, it's absurd to deny that Oda is just doing what everybody else does these days: combining the two birds. The best scenario for you is that Marco is an hybrid of both birds, but that would mean that he has an explicit Eastern side that creatures like the Minotaur lack.