Bic Cristal pens (the ubiquitous ballpoint/biro) come in black, blue, red and green. But according to their website:
What? Why? What are crazy people and teenage girls supposed to write letters with in the US? Is there no demand? Does green ink contain some substance banned in the Land of the Free? What’s up with that? 
These are available. Why buy a single-ink pen when you can get four colors together?
I think this is a question that will be difficult to get a proper answer to, and I believe the reason is just that US Bic Cristal buyers are so uninterested in green ink that Bic has decided to just not sell that there. They will happily sell you a bunch of other green ink pens though.
Seems odd because green used to be the standard color used by auditors.
doesn’t copy in some machines? Probably not the answer.
I guess people will keep speculating wildly, but here’s some further evidence for my hypothesis.
Bic Cristal Grip not available in red or green in North-America
Bic Round Stic - Not available at all in Europe, but “One of the best-selling BIC ball pens in the USA” and available in Black, Blue, Red and Green.
Now why isn’t the Bic Round Stic available in Europe? Round pens banned in the oppressive EU?
I remember those… this oppressed European found them too thin, but whether that’s merely my personal preference or shared by enough Europeans to drive a marketing decision, I cannot tell you.
Cos we have such fat fingers over here 
Bic Crystal Extra Bold - only available in the US - in red and green.
I believe it’s blue ink that doesn’t reproduce well. Thirty years ago, I was on the college newspaper and we used to mark up the pages with blue ink for that reason.
BIC Round Stic Grip Xtra Comfort Ball Pen is available in green, and also purple. So it’s not that Bic pens with green ink are not available in the US, it’s that they don’t sell that particular model with green ink.
Amazon apparently sells some that are imported, though, if that’s what you really want.
delete wrong placement
I’m an animator, and we used those for pencil tests, “Col-Erase Non-Photo Blue” specifically. It’s more of a light sky blue though.
I use purple G2s for my writing, for a variety of reasons including just personal preference, and that’s after switching from years of using green. I gave up on the latter because the ink always sucked and the pens skipped and ran dry much more easily than other colors.
I had no idea they’d become a rarity here. 
Purple G2’s are hard enough to find that I buy them in modest bulk, a couple of dozen at a time, once a year or so. I still have to extract and ‘restart’ cartridges on those, but not as often as green. Funny how the color can make such a difference.
I’m glad green ink is still available here
Banking thing, maybe? They seem to hate green pens.
(Prostitute thing, maybe? They seem to hate green peens.)
That’s the one I linked to earlier, which is apparently an import. It’s “sent from the UK.”
Which means, at worst, that the pens are not made in America, but they ARE available here.
Damn. So all those years I spent smuggling them in in my rectum…