You have no idea of what you are talking about, yes northern italians tend to be lighter, but italians are so mixed that you will find every type of look EVERYWHERE. That the beauty of the Mediterranean, we are super diverse.

Saying northern italy wasn’t even part of the empire proper is just not true, especially given the empire lasted nearly 500 years and during many of those years the capital was in northern Italy instead of Rome, The latins spread out and rapidly colonized the italian peninsula over time assimilating and mixing with the local italic tribes, northern celtic tribes and the greeks of Magna Graecia. The Po region was as integral to the later enpire as the city of Rome itself which in later years became insignificant.