Interesting responses....

Here is the answer I believe closest to the truth:

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.

---------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------