The soft foam earplugs are flesh colored. If not flesh colored, the makeup dept. colors them to match. Many times the plugs are cut in half and pushed deep in the ear so they are not visible
As an armorer you can insist on the actors and NEEDED crew use hearing protection. It is always offered before scenes with gunfire or loud special effects. There will also ( 99% of the time ) be a Saftey Officer on the set that can back you up and also insist on hearing protection. There will also be notices of loud noise, for liability reasons, in the daily film schedule.
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