Making a mailbox is a special experience for me. Most boxes require over 6 feet of soldered joints, and I usually use copper sheet, rod, and tube on each. All hinges are hand made here at the shop. Most mailboxes are completely copper, and some get brass washers hidden in the hinge for strength.
A shiny new copper mailbox will be dark brown in about three months. After a year - it will be nearly black. It will probably will be green verdigris in two years.
Color change depends on weather, and rain accelerates the change.