Photo films and digital sensors do not record colors exactly as seen by our eye / brain vision system.
Initially, photo films were only sensitive to the violet and blue region of the visual spectrum. Translated, films blackened when exposed to violet and blue. They did not blacken when exposed to any other colors. The resulting black & white prints depicted violet and blue as very light shades of gray. Other colors were depicted far too dark. Ladies lips and cheeks with rouge imaged clear on the film and black or near black on the black & white print made from these films. They were called “blind” or “blue sensitive” materials.
Film images often were impaired by a halo like blur that surrounded bright gleaming objects like jewelry or highlight sparkles. These were called “halation” caused by strong light complexly negotiating the film, hitting the back pressure plate behind the film and then reflecting back into the film from the rear.
Professor Hermann Vogel of Berlin Technical attempted to mitigate. He reasoned that halation cause was also blue-violet and could be halted by installing a yellow (blue blocking filter) within the film.
He tried many approaches; in one attempt he dyed the film emulsion yellow. This worked like a charm, halation eliminated. To his surprise this, this emulsion retained the violet – blue sensitivity plus it gained green sensitivity.
This film was named orthochromatic (producing image that better corresponded to nature). With no sensitivity to red light, it could be handled under red safelight. This material retained popularity until just after the WWII.
Vogel’s graduate students experimented with other dyes and eventually discovered dye formulas that extended film sensitivity into the red region. These films revived Greek prefix pan meaning all since they are sensitive to red, green, and blue, the three light primary colors.
Photographs taken blind, orthochromatic, and panchromatic films image colors differently. Additionally it was common practice to mount colored optical filters before the lens to alter the way colors were rendered. Most common was a medium yellow filter initially called a K2. This filter sported a touch of green and was in vogue during this era. Used in combination with pan film, it greatly improved monochromatic rendering. It also darken blue sky allowing clouds to stand out. It would naturally darken blue flag field.
Who knows what film was actually used and what filter, if any was mounted.