In the United States, what is the difference between blue and green mailboxes?

The blue box is the normal collection box for the public. The green box is the drop box, or relay box that a walking letter carrier will use to leave their picked up mail and packages obtained mid-route for another carrier to collect; it also may store some extra forms, extra bags etc. It's for internal use only and saves the carrier form lugging around too much.
https://colors-newyork.com/is-mail-processed-7-days-a-week/

Another answer to this is that originally all U.S. mailboxes were green, then later they changed to red and blue combined, then to mostly blue as they are today.
https://en.wikipedia.org/wiki/Post_box