It's the specific formulation of zinc chromate primer. Zinc-chromate primer is an anti-corrosion coating for aluminum alloy parts for aircraft.
Raw zinc chromate primer is clear and would also give a semi-translucent coating, not very opaque like a pigmented paint or lacquer. Adding a pigment to primer is needed because in aviation, aircraft factory specifications require for just one protective coat of primer, so a proper and complete coating is required.
If another layer of zinc chromate primer is required, each layer needs to be a different colour. Usually, tinting is added to the primer, and manufacturers usually have free reign as to what tinting they want to use.
Usually, this primer is applied after a skydrol resistant finish, which on Boeing aircraft, it is a deep green:
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Below: zinc-chromate primer on the horizontal stabilizer for a Vultee Vengeance:
Zinc-chromate primer with Lamp Black paste inside a A-20 bomber. Adding Lamp Black paste to the formulation would increase UV protection (as zinc-chromate is very sensitive to photolitic reactions) and increase durability in high wear areas. Adding Lamp Black paste would turn the primer anywhere from apple greens to medium olive green.: