That actually was quite common in the European theater. It was also common for officers to have a large diagonal white stripe on the back (so those behind him could tell he was an officer) and NCOs a horizontal one.
Tom Hanks' helmet markings are very accurate for the Rangers who came ashore at Omaha beach on 6-6-44...
Yeah I know about the back of the helmet, but everything I've read said if they did have markings, they would smear mud or whatever to get rid of them. I remember in Band of Brothers Winters said he did that, hid his map case inside his jacket, and then carried a M1 instead of a carbine so he didn't look like an officer. In Saving Private Ryan they acknowledged that they probably wouldn't want that prominent of Captains bars on his helmet or he would have probably covered it somehow. They needed the audience to know it was Tom Hanks though.