Glenn, I apologize for the belated reply: I had completely missed that, I hope it's not too late for some tips
The SLCs were usually finished in two basic colours: black and dark green (I'd presume Regia Marina's antifouling green paint was used for practical reasons -i.e. readily avaiable in large stocks- but i am not 100% sure of it). It's very difficult to give you a precise information about the real colours. Anyway, the brand Model Victoria suggests a basecoat of Humbrol Black Green 91 for its SLC kit. I have checked some italian forums with outstanding navy modellers such as Luciano Rizzato (who posted some great pictures of his "Maiale" on his website
http://www.navymodeling.com): some modellers have tried a base of Humbrol 91, highlighted with Humbrol 149. Some others used Humbrol 77 as a basecoat. An interesting idea would be using the classic pattern that Francois Verlinden used to render the german field gray which is:
Base: slate gray 31+ black 33;
Drybrush: base+white 34+ french artillery green 179
Wash: base + black or raw umber
Final Highlight: french artillery green 179 + touch of grass green 80
But i don't know if this finish is way too gray for a maiale. It Would resemble the "maiale" preserved in Milan's museum though.
The alternative is using the classic Lifecolor UA617 Antifouling Dark Green from LC's Regia Marina original color range which is probably the best choice + playing with highlights and shades.
By the way what's the state of the build right now?