Are you allowed o hunt with an assault rifle in the state of colorado?

There is no legal definition of "assault rifle". If you mean an AR15 style rifle, then it depends on the rifle, and what you are hunting.

An AR15 firing .223/ 5.56mm can be used for small game. For big game, the rifle may not be capable of holding more than 6 rds of ammo, and must be .24 caliber or larger, with 1000 ft lbs of energy at 100 yards. The AR15 in .223 will not meet those standards. A larger AR, such as the AR10, could meet them. No fully automatic firearms may be used for hunting.