For the record and info for those interested, I posted the query about grey vs aluminum undersides at Britmodeller and got the following response:
You are touching a subject that has caused a lot of discussion in the Italian modelling community.. the 3-colour scheme is generally known to have had grey undersides but there is clear evidence that the CR-42s at least initially carried aluminum painted undersides. The reports you mention are part of this body of evidence but there are others.
Some are convinced that Fiat continued using aluminum undersides for longer than they were supposed to do and not only on the CR.42 as the BR.20s seem to have had the same aluminum undersides. Others will deny that aluminum undersides were ever used on the Falco.
Back to proper hard information, it is a fact that aluminum paints from two companies are included in a list of approved paints dated March 1940. A later table (second half of 1940) states that land based camouflaged types must have grey undersides... however in the same document it is stated that seaplanes must have overall grey surfaces and we know that no seaplane had overall light grey surfaces while they generally had aluminum surfaces. It is possible that this relates to the common use of the term "grigio alluminio" (aluminum grey) for the aluminum paint previously used.
The last bit of info is not too clear however there are clearer bits of info: a note sent to SIAI in February 1940 mentions the use of aluminum paint for use on the undersides of military types. Then in Sheet nr. 22/31624 issued by the direction of aircraft production, dated April 1941, it was stated that matt aluminum was to be prohibited from use on exterior surfaces, meaning that it was still in use even if grey was supposed to have replaced this.
Regarding the tables in the Stormo website, they are not official tables but are the result of the hard work of a number of researchers who started publishing their results in the mid '70s. They are extremely useful but all these schemes are classified for ease of interpretation by the modellers, there never was a scheme known as C1 in the '40s. Most information on the use of aluminum after the start of the war was unearthed in more recent years.
Based on the evidence above and discussions with other modellers who studied the matter more than I did, I believe that aluminum undersides were used for longer than previously thought. For a BoB CR.42 I would most likely go with this colour rather than grey.
Decision is final. I'm going with aluminum undersides.