Brumbles, you and I are in the same boat as far as getting back into models.
Has the olive gray been accepted for the Model 21 Zero? Yes and no.
I used to edit the IPMS-USA newsletter for the Japanese airplane SIG. A couple of years before I took that on, Mr. James Lansdale (J-Aircraft.com) published a paper in it circa 1996 about the color. Since then he has revised his essay several times, generally making it better. He was probably the first Westerner to publish color measurements, but the discussion had been going on in Japan since the 1970s. Sometimes with scathing comments.
If you look at Japanese commercial art of the last 20 years, you'll see varied interpretations. The 2012 film Eien no Zero (永遠の０) portrays the plane in something roughly akin to the FS 26350-ish color that Mr. Lansdale found repeatedly on relics. Yet many Japanese modellers have not really embraced it, or go just part way toward it in their color selections. Likewise, Western modellers still have varied interpretations, with some being extreme.
Personally, I believe that the relics have yellowed somewhat over the years. If you know paint, then you know what yellowing does to gray. (It moves it towards greens.) Some yellowing is likely when the formula contains a lot of white, as these controversial light and medium colors do.
One thing Mr. Lansdale failed to discuss in his earliest writings was that the color of the paint varies a good deal depending on lighting and angle. At some, it is rather green. At others, it has some warm brown with barely any green. In other words, you are likely to get fairly close to a metameric match (a match for some purposes but not all) if you work at it, but you'll probably never get all the activity that was in the real paint. I doubt that any model paint maker has duplicated that aspect of it.
If you think of the boundaries for the color, we know there is some brown or warmth to it because of the word "ameiro" being used in a JNAF report to describe it. We don't know from that report how much ameiro there was. Some may be the effect of paint aging, but not all. The officer said it had a little ameiro in it when it was new.
The Japanese have several interpretations of the word ameiro, but none of them is a green. Think of yellowish-tans, ambers, burnt oranges, honey, etc. So, the greener one makes a model, the farther he gets away from the officer's description of the Mitsubishi Zero as of 2/42.
The handling manual calls the color nezumi iro, which also is not a green.
So, your model probably should not be the color of pistachio ice cream or a celery stalk, unless you wish to depict the green-extreme of the paint or something unusual. Likewise, you probably do not want your model to be beige.
You raise the traditional Japanese color name "ameiro". As far as anyone knows, that word was used just the one time by the one officer to describe the color. The Japanese have some different interpretations of that color name, but they are not greens. For example, see "ameiro tamanegi" (carmelized onions), making a Google search with these kanji: 飴色たまねぎ Ameiro tamanegi is a round onion that is sauteed, sometimes up to 20 minutes.
What is a little amusing about all the ink on this subject is that most of the Western writers have never personally seen a new Model 21 Zero. And neither have I.