Back in the 1940s as well as in the paint industry of today, the term Zinc Chromate does not refer to a paint color, but rather a protective coating.
Zinc Chromate is a corrosion resistant agent that is added to certain coatings. Even today, chromate finishes including Zinc Chromate provide superior corrosion resistance. Additionally, Zinc Chromate is highly toxic thus protecting the surface from proliferation of organic matter.
In the aircraft industry of the 1940s, Zinc Chromate was used as an anti-corrosive barrier primer; it could be described as a sort of painted-on galvanizing. It has been developed by Ford Motor Company by the late 1920s, subsequently adopted in commercial aviation and later by the US Military. Official USAAC notes mention successful application of Zinc Chromate primer starting from 1933, but it has not been adopted as standard until 1936.
Because Zinc Chromate is all about corrosion protection, the precise coloring of it is and has not been considered as important as the chemical composition. In the official notes of the period, the name Zinc Chromate is often accompanied by the name of particular manufacturer, thus mentioning Ford Zinc Chromate, DuPont Zinc Chromate or Berry Brothers Zinc Chromate. This means that the actual color of Zinc Chromate coating may have varied from batch to batch or manufacturer to manufacturer without it being viewed as an issue.
The 'native' tone of zinc chromate crystalline salt is a bright greenish-yellow. When put into a vehicle with binders to make paint, this color would be the raw result.
Such raw Zinc Chromate primer would also give a semi-translucent coating, not very opaque like a pigmented paint or lacquer. This property becomes especially interesting when we consider that aircraft factory instructions often called for just one protective coat of primer. As a consequence, the color of the underlying surface might have a significant effect on the final appearance. For example, raw Zinc Chromate applied on the white background would look yellow, while applied to bare metal aluminum it would look more like apple green.
Similarly, any pigment might be added to the raw paint mixture to go with the Zinc Chromate, thereby modifying the color. Some of today's mixtures use iron oxide -- giving that rusty red appearance you can often see on prefabricated steel beams in highway and building construction.
So what does all this mean? Perhaps no more than there hasn't ever been any specification in the industry for a Zinc Chromate color. This in turn caused alternative designations to pop up in the literature that attempted to describe the color value of the Zinc Chromate finish - Zinc Chromate Yellow and Zinc Chromate Green being the prime examples