Hi Pete,
Very interesting project...and the work in top notch!
In Eberhard Moller book "U-boats fron 1904 to Present" there are a couple
of photos showing S 170 and S171 on the surface. These boats seem to
be relatively dark in shade when compared to other photographs in similar
light conditions.
In Rossler's book "The U-boats..." both S-170 and S-171 sport a light gray
in a photo referring to their re-commissioning but once the modifications
were made to these boats, as seen in another photo, one can distinctly see
a dark gray shade.
In my opinion you would be correct to paint your variants (post war) in a
dark gray shade. One model of a post war U-boat I am building suggests
"Panzergray" as a appropriate shade.
Hope this helps,
Have a nice day and happy modelling, 
 
 Joe