Past Insignia
The current Air Force officer rank names and insignia were borrowed from the Army upon the establishment of the Air Force as a separate service in 1947. This explains why the Air Force has brigadier generals but no brigades. The insignia have been essentially unchanged since then, except for a brief period during the 1990s.
The Army origin explains why the color silver outranks gold in Air Force officer insignia. In the early 19th century, Army infantry colonels wore gold eagles, while all other Army colonels wore silver eagles. When the Army later decided to adopt a single color of eagles for all colonels, that single color was silver, apparently because more silver eagles than gold eagles were already in use. Lieutenant colonels received silver oak leaves next, to match the silver eagles of full colonels. Majors were then given gold oak leaves to distinguish them from lieutenant colonels. Once the precedent of silver outranking gold was set, it was followed when gold bars were later created for second lieutenants, who had no grade-specific insignia until the early 20th century.
During the 1990s, Air Force Chief of Staff General Merrill A. McPeak redesigned the service dress uniform to give it a more distinctive look. His redesign replaced the metal rank insignia for officers with silver braid on the sleeves, similar to the Navy's officer rank insignia and that of the United States Coast Guard's. This was done in the traditions of the British Royal Air Force, the Canadian Forces and the Commonwealth air forces. The "McPeak uniform" was very unpopular, drawing comparisons to the jackets worn by airline pilots, and the traditional rank insignia were reinstated within a week of General McPeak's retirement in 1994.
Read more about this topic: United States Air Force Officer Rank Insignia