DC Members,
    I talked to a couple of retired experts (my brothers )  and this is what
they had to say....

First my brother John:

My theory is that the color green preserves the solder coating on the
board from breaking down, a lot like the color brown on beer bottles
keeps the beer fresh.  I could give you a long theory to go along with
that, but I need to delve into the color brown right now.

and from my brother Joe:
(p.s. shows where the brain in the family is....)

I thought that everyone knew the answer to this question.

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.

Hope this clears this matter up.

: )

Karl