I live in Colorado and Goose hunting is mainly done in the fields but the vast majority of duck hunting is done on the water. I moved here from Mississippi and thought I'd just come out and hammer the ducks and geese in the fields and that is sadly not the case. If you want to get on a good field duck or goose hunt you will need to have private land or some sort of lease as the public properties get hunted every single day by the biggest sky busting idiots you will ever see. There is also very little duck huntable public land out here. That being said, Colorado probably has some of the best dry field duck hunting in the country.
Keep your diver decoys. We have high numbers of divers all season from the start of October through the end of January. LOTS of redheads and ruddys early on, then you get your ringnecks and bluebills which never leave along with some Canvasbacks, followed by your buffleheads which tend to hang around til the end and then finally your Goldeneyes which are everywhere once things start to ice up around Thanksgiving and those will hang around til the end of the season. Also, if you're the kind of guy that is into the whole shooting a limit of mergansers kinda deal then Colorado is the place for you as they are everywhere
Lastly, there are a lot of mallards out here and by a lot I mean ALOT. Just a heads up.
There's great hunting to be had in Colorado but don't get your hopes up on field hunting ducks.