The Proteo Red is a beautiful colour, but it would look strange on a Duetto in my humble opinion (or rather "in my taste"). The tender lines of a Duetto would drown in that dark red, but on a more muscular-looking car it would be fantastic. Again, to my taste.
How about the flat AR521 Rosso (dubbed "lipstick red" by a fellow Alfista) you find on Montreals? It's darker than the usual Alfa Rosso, but still very red.
Most of the new Alfas I've seen the paint sticker of bears the DuPont logo, but I've been told any car painter can mix any colour as long as he knows the original colour code.
Anyway, the Alfa Romeo 159 has a colour called "AR583 Rubino Red":
