stevef
It probably doesn't apply in this case but during the filming of The Dambusters at RAF Hemswell, screen 'officers' wore brown shoes to distinguish themselves from RAF real officers. This was to avoid confusion and airmen saluting them by mistake. The different colour didn't show up in the black and white film.
Apparently Richard Todd - who had held a commission and was (possibly) still a reservist of some sort - insisted on wearing black boots/shoes and getting the salutes!