Looking at the photos, I was seeing a brown-green but thought it was a trick of light and a color I have never seen. I looked up '73 as SeaDog was posting and noted "no green" that year, but also the brown I was seeing in the pictures could not be from the colors available such as the saddle or that Madrid brown. Seeing as how he confidently pointed to a '71, I thought I missed something and left it there. ...but I see you also pulled up '73.
The brown I thought I saw mixed in the green is a dark brown. To be honest, I've only seen it in the '80s Pontiac Grand Prix I had which was champagne in color. I just tried to look up the color, and the color swatch I see that I consider brown is black. ...even tho it looks brown compared to all the other blacks on the page.
Maybe it was ?black? and some dye faded leaving that greenish color? ...but 255 is "saddle", and I can't say I've seen saddle or black for that matter fade into a color anywhere near that.
I'd say you definitely have a mystery on your hands. It is no known color.
That Grand Prix brown (black) looks like this: https://www.carid.com/cipher/1-yard-x-57-leatherette-seat-material-mpn-cpa9300pbk.html?singleid=372372541&vehicle=7405 my wife sees that as brown too, so, maybe black with QA issues in the dyes.
To put that black in contrast, second one, "matte finish", https://www.carid.co...ive-upholstery/
Either way, that isn't what the body tag says it was or should be.
Given the lack oh PHS documentation and the odd color, I might suggest it is a "pre-production" or a very early production model. ...which is "rare".
Definitely hold off on a color change until you're positive what you have there.