Unfortunately, there is no definition of what exactly constitutes "concealed."
IANAL, but in People ex rel. OR the court found concealed to mean "placed out of sight so as not to be discernible or apparent by ordinary observation." Under this definition, carrying a handgun in a holster that is otherwise not concealed does not constitute CC. A holstered firearm that is worn openly is OC. Denver would certainly view it that way.
There have been a few discussions on here about exactly how much of the firearm can be covered by the holster itself but there is nothing in the statutes about this. I would think that pretty much any holster is fine except those that cover the entire firearm (like some military holsters). But even this is unclear.
Generally speaking, an uncovered firearm is OC. A firearm hidden from plain view is CC.