Italy’s new Green Pass

https://ambwashingtondc.esteri.it/ambasciata_washington/it/sala-stampa/dall_ambasciata/2021/08/aggiornamento-ingressi-in-italia.html

click on ENG , at the top right of the page

Hi there,

Italy's green pass regulates who can participate in certain social activities, including indoor dining, cultural exhibitions and monuments, and theatre performances and concerts. Vaccinated travelers need to show proof of vaccination. So for you coming from the US, that is most likely your CDC white card. 

Currently there is also another entry requirement which is proof of a negative Covid test at least 72 hours prior to arrival in Italy.

If you aren't vaccinated, there is currently a 5-day quarantine for US passengers after arrival, and you'd have to take a Covid test after quarantine and then every 48 hours to enter sites where the green pass is required.

The website italygreenpass.com has a lot of good information in English with links to official sources. 

Hope this helps!