Hello Glenn!
I've been wondering about something like that for some time, too! From what I've seen some of the Herks have been natural aluminium finish or painted silver. most of them probably before '65 when they introduced the SEA camouflage.
I dug up an olf Tech order 1-1-4 and it says "All exterior areas except as specified" and then "Former system" colour 17178 and "Standard system" 16473 which I'd say are numbers also applicable to the FS-595. The first one would be silver, the second one ADC Gray.
Hope this helps - have a nice day, and by the way, Happy Easter everybody!
Paweł