All the vermouths (except for a few oddball contemporary ones) were and remain fortified white wines. I’m a bit unclear when it happened, but at some point in the early-ish 20th Century—for marketing reasons—the Italian producers began coloring their core products with significant caramel coloring and eventually adopted the “rosso” designation, to differentiate it from the dry and bianco styles they adopted around 1900 and 1910, respectively (the years vary a bit from producer to producer, but those are the years for Martini & Rossi). Somehow, this dramatic change in coloring didn’t bother many people, and it quickly became the norm. (I wonder if it happened during our Prohibition, and that’s why it was a non-event?) For early recipes calling for Italian vermouth, you want rosso, even though the resulting drink will be darker than it would have been at the time. For example, a 1880s Manhattan would have been much paler than we are used to seeing today.
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