In the United States Air Force, including the Air Reserve Component (Air Force Reserve and Air National Guard), gold represents the lower form of a rank.
A gold bar ranks below a silver bar. A gold leaf ranks below a silver leaf.
It seems to me that the military takes its symbols very seriously and thus I'm wondering what the rationale is for this, given that in the civilian world gold generally symbolizes the best form of something (or at least better than silver).
NOTE: I've only been focused on the Air Force, however I just noticed this applied to all of the US military.