Here is the answer from the previous TechNet discussion:
(Ioan, your memory is not so good...., grin)

The green color of soldermask was chosen after extensive testing by the U.S.
military at the National Materials and Procurement Center in Cedar Bluffs
Virginia in late 1954. According to Colonel Robert Bright, Public Liason
Officer, the particular shade of green was found to provide the maximum
contrast to the white silkscreen ink under all tested adverse situations
while still allowing a clear view of the underlying circuitry. Every other
color tested failed to provide the required contrast under conditions of low
/ high illumination levels and various colored light sources.
The quantity of Printed Circuit Boards purchased by the Defense Department
established a "de facto" standard which most vendors began to follow. Usage
of other colors is permitted for prototype and or pre-release boards.

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