Like other arborvitae, Thuja
Green Giant should do OK in the Denver area if planted in full sun to part shade. The down side of arborvitaes is their tendency to winter burn in growing areas with low winter humidity and strong winds, such as in Denver. This tree is a hybrid arborvitae (Thuja
) introduced by the U.S. National Arboretum. It is fairly uniform in shape and hardy to Zone 5. Ideally, you should plant them where they will be protected from prevailing winter winds out of the west and north. Winter watering during extended dry spells would be helpful. They may be damaged by heavy wet snows.
If you have more room, consider Southwestern white pine (Pinus strobiformis
), which has relatively fast growth for an evergreen, to perhaps 50 by 25 feet. See also http://www.ext.colostate.edu/pubs/garden/07403.html