Yep, it's legal under Colorado state law, and the state's preemption statute makes that legal in the jurisdictions of local governments as well. Carrying a long arm openly is even legal in Denver (their confusingly-worded law can be found here: http://library1.municode.com/mcc/tem...&doc_hit=1 ), as their special status to restrict open carry only applies to handguns.
Just keep in mind that carrying a long arm is much rarer and potentially much more alarming than a handgun in a holster. I'd keep it on a sling if I were going about on foot, and I wouldn't linger in a business on the way unless you're comfortable with it. Muzzle control is the key!