I want to precise that the request I received concerned Fiat G55, aeroplanes produced after the introduction at the end of 1941 of the use of Tavola 10 colors, so the mentioned rules are applicable following that date.
I attach an image of Cr42 used as night fighter, where it is possible to discern a small part of the internal of the cockpit and it seems to be Grigio Azzurro Chiaro.
With reference to the period before the introduction of Tavola 10, we know from the G50 manual that Fiat used a grey anticorrosion primer.
The only known grey primer is the one found an a part of S79 and it is similar to FS 26599; so we can say that probably the primer had this shade, but up to now no certainty exists.
So it depends on the perid of the represented model.
These are all the information I can provide, I hope that in the next future some other information can arise
Riccardo