There is another excellent photo of a brand new GN #71903 on page 31
of the Four Ways West Great Northern Freight Car Pictorial Volume 2.
This photo is not in the RP Cyc. The car looks very new, and the
lettering (ALL of it, including the herald) definitely looks "other
than black" to me. I would actually have guessed bright red, if Steffan
had not found the drawing that says it was "box car" red.
We know that black & white photos can be tricky to interpret, and I'll
happily stand corrected by defering to painting/lettering diagrams if
they can be located. The AFE may well be correct about the use of box
car red stencils, but on gray Airslide cars built in 1954-1955 this
color would be pretty unusual.