Film Colorization.https://en.wikipedia.org/wiki/Film_colorization
Core of it is a known reference color. Digital method software used to colorize film or pictures relies on a common reference color to key the gray scale. From there it is the operator's eyeball while tweeking the converted grayscale to color results.
Fake.There was no clolor in the world in 1942. I have tons of photographic evidence for that.