can we say "good morning"at any time in a day?
Good morning can be said at any time of the day as long as you are seeing someone for the first time in a day irrespective of the time of the day.
Good morning specifies the beginning of a day/thing or a meet with anyone according to the principle of English, you can greet good morning to people you met for the first in whole day irrespective of the time be it afternoon, evening or night.