According to the book Stanley Kubrick: A Biography by Vincent Lobrutto, the accuracy of Full Metal Jacket is so complete that Kubrick filmed in a part of England that had been designed by the same architects that had been used to plan the industrial areas in Hue where the battle scenes took place, the area was essentially the same in layout as those that had been depicted in Gustav Hasford's book The Short-Timers that the film was based on. There are many pictures too that bear out that soldiers did indeed take to making their uniforms more personal. This is a famous photo of a young man that did just that.

I would note too that Kubrick was a meticulous director that worked very hard to ensure that his films were visually in line with the historical periods they depict. Indeed he adapted lenses used in spy satellite photography to be used to capture natural candlelight in the film Barry Lyndon. In Full Metal Jacket since he refused to fly he not only scouted a location that was faithful to the architecture in Hue he imported hundreds of palm trees so that he could provide realistic landscape in the countryside.

Edit, here is a printerest page that has a lot of photos that should interest you.

I have a follow up question if it's ok with OP and mods.

In the scene where the Colonel confronts Joker about the "born to kill" on his helmet and the peace pin, how realistic is it that he saluted Joker first?

I viewed it as trying to get Joker pumped up about the war, but it just didn't sit well with me.