So the Sanborn map keys show that many of the 1950-era roofs were "composition" roofing. I usually think of this as asphalt shingles, but would these have been used on an old warehouse, or would rolled roofing have been more common?
The Forest Service wrote a nice document on Early 20th-Century Building Materials: Siding and Roofing which states that asphalt roofing existed in the 19th century, but really took off in the 1920's to cut the fire risk of wood shingles. Asphalt shingles were popular by World War I because they were easier to transport. Colors for asphalt roofing with embedded aggregate was limited to black, red, and green until the 1930's when ceramic granules of other colors started appearing.
So, it's a toss-up - shingles or rolled roofing? Time to keep researching.