Are you sure that's a bar?
But, generally for CO, a "no guns" sign has no weight of law, and it is legal to carry into an establishment that serves alcohol.
It is common in many places for business to imply that their sign has a law behind it, even when it doesn't. Be especially wary when they do not provide a citation for the law on the sign, though often this is also done with a misleading citation.
To clarify, I do not advocate carrying into places with such a posted policy once you know about it. (I also do not advocate going out of your way to look for such signage). I do not know the Denver area, but where I am in NV, we have just as many pro-gun bars as we do anti-gun ones.