Welcome to the forum.
I've never heard of any legal restrictions on holsters in any state, but I could be wrong.
Colorado statutes say
Further, case law dictates that
(1) A person commits a class 2 misdemeanor if such person knowingly and unlawfully:
(b) Carries a firearm concealed on or about his or her person; or
I am not a lawyer, but it seems to me that the average juror would not convict you of unlawfully carrying a concealed weapon for carrying in any holster which allows the fact that you have a firearm to be discernible to ordinary observation. To me, this would be any holster which exposes any part of the grip and/or one that has a gun shape to it. I probably would not carry in a fanny pack, camera case, or anything else which would prevent the fact that I had a firearm to be discernible or apparent by ordinary observation.
"Concealed" means placed out of sight so as not to be discernible or apparent by ordinary observation. People ex rel. O.R., 220 P.3d 949 (Colo. App. 2008).
Question of whether weapon is concealed is question of fact for the jury which should not be summarily determined by the trial judge at the time that he rules on the defendant's motion to suppress. People v. Vincent, 628 P.2d 107 (Colo. 1981).
Of course, it goes without saying that none of this applies if you have a permit.