I’m in Chicago and most of my team is in Paris, so this is a situation I have a lot of practice with!
My primary recommendation is: reference the time of your audience.
However, the key fact is: anything you say nicely is fine.
Typical conversations I’ve had at 8 AM my time (CST), 3 PM their time (CET):
Paris: Good morning!
Chicago: Good morning! uhh . . . Good afternoon!
Paris: Good morning!
Chicago: (Yes, it is a) good morning!
Paris: Good morning!
Chicago: Good afternoon (to you)!
These generic expressions of goodwill are fairly flexible. Primarily, we are wishing that people hearing us are enjoying whatever time of day it is. The same words can also acknowledge their wish for us, or even confirm that we are enjoying whichever time of day it is. That means whatever you say, you are justified, but generally it is more polite to acknowledge the reference frame of the people you are speaking to.
Remember, all this etiquette is rooted in practices that predate international conversation by a long shot. This is something that is acknowledged as awkward among native speakers and we laugh about it.
It sounds like your coworkers are treating this type of statement primarily as a report rather than a wish. They are saying “(it’s a) good morning (here in the US)!” This means you have two polite options! You can adopt their sense of the greeting and report from your own locale: “Good evening from here!” OR you can politely wish that they go on having a good morning in their location! Either’s fine, or even both!
A short greeting that was a nice way of saying "Hello."
a phrase used during the day as a greeting or farewell
- I'm not the only one that considers it a greeting as well as a farewell. I think that question has as many downvotes for the "streetspeak" section.