Technically, green peppercorns are from the same plant as black or white peppercorns. Green peppercorns are unripe black peppercorns, and white peppercorns are black peppercorns with the outer coat removed.
Green peppercorns are true peppercorns of the Piper nigrium flowering
vine plant.... Green peppercorns are really unripe black
peppercorns. These are often preserved in brine or vinegar and served
in pickled form. In dried form, they don't last very long and have to
be used quickly. They're commonly found in Thai and other Southeast
Asian recipes and have a fresher flavor than their black counterparts. (via Kitchn.com)
Most steak au poivre that I have had (never made it myself) had visibly black peppercorns, coarsely cracked and liberally coating the outside. I would suggest using that while you're perfecting cooking technique, and then try using pink (or if you can ever find them, perhaps in an Asian market, green) to see whether and how much it impacts the flavor of the dish.
One potential substitute is capers, which are also preserved in brine (and if I didn't know better, I'd think the dish in your Question's picture had capers in the sauce), but the flavor is likely to be fairly different.