In the late 1920's, the Ford Motor Company
developed an anti-corrosive primer using a
chemical called Zinc Chromate. The U.S.
Army Air Corps tested this paint on aircraft in
1933, and adopted its use as a standard practice
in 1936. By 1940 it can be found in widespread
use by railroads.
Zinc Chromate is not defined as a color, but rather an ingredient added to
paint. Thus the actual shade and tone of paints with this ingredient will
vary. In its pure form, zinc chromate is mustard yellow, is mostly translucent,
and does not provide ultraviolet (UV) protection. To make it opaque,
more durable, and boost UV resistance, a tiny amount of lampblack was
added. This did not turn the mix “brown” as you may expect, rather the
familiar “sea-foam” green was the result. Varying the amount of black in
the mix produced shades from apple green to army camouflage.