During the entire Viet Nam segment, the Marines are shown wearing full leather boots (all black), at that time standard issue was the leather and nylon jungle boot (green and black).
This is a slip-up, though a deliberate one. Stanley Kubrick, was told by R. Lee Emery that the boots were not the correct type. However Kubrick got a great deal on Dutch issue all leather combat boots. The slip up was a way to keep the costs of the film down and stay under budget.
This isn't true. I was in the marines from 69-72 and we wore the black leather.
Back then the marines were known for slowly replacing items and until they had enough for the whole unit no one used them. In short we wore the black leather