I agree with @ARGTAReg
that Rosso Rubino Scuro and Rosso Amaranto would be the closest candidates -- as a starting point.
I would suggest the consideration of the following three caveats:
1. I do not believe that a race car (like the Spider shown in the first post) would have been painted in that color (but rather the Rosso Alfa race color of the period).
2. I do not believe that the was a uniform or standardized Alfa Romeo color palette at the time, because (a) paint was applied (and color decided) by the coach builders, and (b) color was mixed in such small batches that there probably was quite a bit of variation even of the same color, and here was likely some variation in painting processes (who painted it, temperature, humidity, etc.).
3. Ian, who painted the 8C 2900B that won Best of Show at 2018 Pebble Beach (see here
), told me that he painted over 100 test panels until they figured out the shade of dark blue color, and how to reproduce it repeatedly (as the restoration workflow dictated that the body and fenders had to be painted at different times). I recall him showing me three test panels that came from the same gallon of paint (with same prep, dilution, solvents, etc.), but, under test lights, looked different -- just because of different drying times (which may have been due to small variances in temperature, humidity, moon phase, etc).
So, I would not go overboard with research on original color: First of all, I don't think there are any reliable sources what the original color was. Secondly, the chemistry of paint has changed quite considerably since then. Thirdly, the quality standards of what we consider to be good or perfect paint are substantially higher now than they ever were in period (for example, the paint job on the car shown above is now probably much better than when it left the factory).