Never heard of anyone needing an undercarriage coating in Colorado. (lived here for 50yrs) CDOT does not use salt on the roads, although they do use magnesium chloride, which can be corrosive. Unless you are trying to stop existing rust acquired from another region, I am not sure you will need an undercoat in Colorado. I just use that 60 degree day in Dec-March to get to the carwash as soon as I can and power wash the winter road grime off my cars.
Colorado mag chloride is just as bad as any road prep crap used in the rust belt, though they only really started using it heavily about a decade ago, and we don't get as much precipitation as they do out east, so it's not applied as often. I've fought stuck rusted bolts in 2-3 year old vehicles that have never left Colorado and washed regularly. Yes, this is becoming a Colorado thing.
I wish they would just plow and issue tickets to anyone found lacking in the winter tire department.