The armbands were required by regulation. The helmet markings were not. At Normandy, most front line medics did NOT wear them, as they were not yet convinced the German would not target them.
Fairly quickly, the US troops learned that on the whole, Germans did not target medics, and it became the norm. You see a great many styles for a while, (squares, circles, 3 or 4) but eventually the 'gold standard' was red cross in white circles on the four sides- and the medics red cross number along the top in white.
During the transition period, it was very much a unit decision. The medical officers decided what to do, and had the helmets painted. Sure, a guy could decide he did not want to have a painted helmet, but it very quickly became apparent that it was a lot safer to have the marks. Some medics went to wearing an armband on each arm- but that was very much a personal preference.
As an aside, my dad was badly wounded in an action when his company was trying to breach a German defense line in Brittany. As they tried to push forward, a US medic was shot (apparently by accident) and this so enraged the men they surged forward. The Germans fled and a local farmer living there later reported the Germans cried out as they fell back "a medic got shot- they aren't taking prisoners".
But the 82nd was in Italy before Normandy, and I have not looked into them much. It's very much a 'look at every photo you can find of the actual time period' and the exact unit.
paints a machinegun emplacement, and realizes he needs more...
