The Colorado Division of Motor Vehicles (DMV) notes that non-residents may drive using their current, valid driver's license issued by their state or country of residence. They also note however, if you take a job in Colorado or live in the state for longer than 90 days, you will be required to surrender your original license and apply for a Colorado drivers license.

So it appears your Italian driver's license would be valid for the 90 days you are going to be in Colorado waiting to get a green card and you can contact the CO DMV to see if after 90 days you can continue to drive on your foreign license or after what point you will be considered a resident and can apply for a US (Colorado) license.

Remember it is your Italian license that you will need to drive on, not just an International Driving Permit (IDP) since this is only a translation of your license and is not valid as a license. It is helpful to have an IDP though so if you need it you can show it to law enforcement.

As for insurance if you are planning on driving your fiancé/ husband's vehicle than he will need to speak to his insurance company to see if they will cover you when you are still have your Italian driver's license and have not yet received your US driver's license. Insurance company guidelines vary so his insurance agent should be able to tell him if you will be covered and if so for how long. If they do not cover you under his policy than they may be able to advise your fiancé how you can obtain car insurance while you only have your foreign license to drive on.