While the state of Colorado doesn't explicitly prohibit gun owners from entering private buildings, it does have criminal trespass laws (
C.R.S. § 18-4-504) which allow property owners to decide to forbid firearms from their property.
MySecuritySign offers No Guns signs for Coloradans who don’t want firearms on their premises.
Colorado Gun Control Law Sign : No Firearms On The Premises - §...
Part #:S2-0646-CO 10" x 7" HDPE Plastic Sign Verified PurchaseOutdoor use
Nice sign