Fortunately, there is a LOT of public land available to hunt in Colorado (Wyoming, Montana, etc.). Unfortunately, it's not "right next door". The front range (Colorado Springs - Denver - Ft. Collins) is largely private. Some of the countieshave purchased bands of property for use as "open space", but last I heard - hunting wasn't one of the available uses (thank a relatively largepopulation of anti-hunter/"environmentalists").

Drive a couple hours west and you'll find plenty of National Forest. Of course the drawback to hunting public land is that it's open to EVERYONE to hunt, so the easy accesses get congested quickly during hunting season.