'STAR WARS' AT 40: "ALL PILOTS TO YOUR STATIONS"

As Luke slips into his fighter cockpit with ease and youthful anticipation, the final Rebel pilots get ready for flight. For this scene, filmed in May 1976 at Shepperton, the unknown pilots were played by background extras who wouldn't be used with the later filmed cockpit interior scenes at Elstree the following July.

Note that the very clean looking helmets they wear all have the Rebel insignia in the blue colour, part of the Blue Squadron of fighters- all from a set of helmets produced en masse for the prior filmed medal ceremony finale crowd scenes, filmed, as here, at Shepperton Studios. Due to ILM's worry about blue screen overspill during filming/optical compositing, however, Blue Squadron became Red Squadron several months later and the helmets were adapted to a wide variety of colours and designs instead, importantly helping to visually single out the various pilots in scenes. Intriguingly though, Luke's Red Five call sign pilot helmet at Shepperton clearly has the rebel insignia seperately in red on his helmet (as seen in all the production photos), but by the time of the Elstree cockpit filming another helmet must have been used for the actor- Luke's dirtied-up insignia helmet in HD clearly shows that it has been altered from blue to red.

Mark Hamill in the cockpit at Elstree. Note the back of the soundstage behind him.

The original ILM X-wing fighter models that had blue stripes indicating their squadron colours also changed with speed States-side... https://starwarsaficionado.blogspot.co.uk/2016/12/rogue-one-blues-in-battle.html

One of the original Blue Squadron X-wing models at ILM.