Checking few pictures of SM79s and Italian torpedos, it really seems there's no real standardization in terms of colours.
It is possible that the "head" was painted in red sometimes, but I wouldn't draw any rule, IMHO.
Here's an interesting pict of a SM79's torpedo that I found online. It's taken at the Vigna di Valle Museum.
http://lh4.ggpht.com/_yBKu6IwRjCY/Sf...0/DSCF1139.JPG
Although I would not follow the pict for the red nose as a rule, the grey colour looks quite realistic to me. Should I judge from the war picts I would say that the colour was either grey (the ordinary "grigioazzurro"?), either bare metal, maybe depending upon the producer. I guess that Veltro will be able to collect all the needed information on the Italian forum.
EDIT: one more pict.
http://media.photobucket.com/image/s...rviero-003.jpg
The nuance of gray looks way too lucid to me, in this restored piece, although the gray tonality doesn't look so bad.