Colo. Rev. Stat. § 13-25-117
In trials of actions upon contracts, expressed or implied, where the action is brought by partners or by joint payees or obligees, it shall not be necessary for the plaintiff, in order to maintain any such action, to prove the partnership of the individuals named in such action or to prove the first names or surnames of such partners or of joint payees or obligees, but the names of the partners or joint payees or obligees shall be presumed to be truly set forth in the declaration, petition, or bill. Nothing in this section shall prevent the defendant from pleading in abatement or proving on the trial that more persons should have been made plaintiffs, or that more persons have been made plaintiffs than have the legal right to sue, in which event the defendant's right shall be as at common law.
C.R.S. § 13-25-117