I wouldn't carry any normal capacity magazines into Colorado. I wouldn't transfer nor accept the transfer of a normal capacity magazine in Colorado.
As with the many efforts to criminalize handing a firearm to somebody else I don't know what
transfer means - in Washington state I can't legally hand a shotgun to my shooting buddy while I climb a fence and he can't legally hand it back to me after I'm over nor can I legally hold his gun while he climbs the fence - I'd be leery of having even a known legal to me normal capacity magazine in anybody's hands during a group shooting event.
I'd read the language of the statute in addition to asking advice on the errornet
18-12-302. Large-capacity magazines prohibited - penalties - exceptions
(1) (a) Except as otherwise provided in this section, on and after July 1, 2013, a person who sells, transfers, or possesses a large-capacity magazine commits a class 2 misdemeanor.
(b) Any person who violates this subsection (1) after having been convicted of a prior violation of said subsection (1) commits a class 1 misdemeanor.
(c) Any person who violates this subsection (1) commits a class 6 felony if the person possessed a large-capacity magazine during the commission of a felony or any crime of violence, as defined in section 18-1.3-406.
(2) (a) A person may possess a large-capacity magazine if he or she:
(I) Owns the large-capacity magazine on July 1, 2013; and
(II) Maintains continuous possession of the large-capacity magazine.
(b) If a person who is alleged to have violated subsection (1) of this section asserts that he or she is permitted to legally possess a large-capacity magazine pursuant to paragraph (a) of this subsection (2), the prosecution has the burden of proof to refute the assertion.
(3) The offense described in subsection (1) of this section shall not apply to:
(a) An entity, or any employee thereof engaged in his or her employment duties, that manufactures large-capacity magazines within Colorado exclusively for transfer to, or any licensed gun dealer, as defined in section 18-12-506 (6), or any employee thereof engaged in his or her official employment duties, that sells large-capacity magazines exclusively to:
(I) A branch of the armed forces of the United States;
(II) A department, agency, or political subdivision of the state of Colorado, or of any other state, or of the United States government;
(III) A firearms retailer for the purpose of firearms sales conducted outside the state;
(IV) A foreign national government that has been approved for such transfers by the United States government; or
(V) An out-of-state transferee who may legally possess a large-capacity magazine; or
(b) An employee of any of the following agencies who bears a firearm in the course of his or her official duties:
(I) A branch of the armed forces of the United States; or
(II) A department, agency, or political subdivision of the state of Colorado, or of any other state, or of the United States government; or
(c) A person who possesses the magazine for the sole purpose of transporting the magazine to an out-of-state entity on behalf of a manufacturer of large-capacity magazines within Colorado.
If it mattered to me I'd ask the advice of an attorney chosen for knowledge of firearms law.
It is my clear understanding that possession of normal capacity magazines is grandfathered in Colorado. I suspect that means that a normal capacity magazine that has been continuously possessed by the same person in Colorado since before the implementation date of July 1, 2013 won't get the owner in trouble - I suspect in Denver, city or county, that safe harbor might not be so safe.
Given that transfer is a violation previously made magazines can't be subsequently transferred for use in Colorado. The law is not specific enough with respect to moving into Colorado or passing through with a stop. As noted there have not been enough cases to establish a useful stack of precedents so I'd avoid being the first and discovering what the law means that way.
Myself in Colorado I make do with magazines of less then the listed 15 cartridge capacity. Mostly 10 count so there is no issue of jamming one more in with a different floor plate or follower.
If it mattered I do have some Colt marked normal capacity magazines dating from the futile Federal ban period. The magazines are serial numbered, dated and marked for LEO only. I could prove the date I got them to a reasonable moral certainty.
I have a box of 10 normal capacity magazines from Brownell's that were in my possession in Colorado before July 1 2013 and a box of 10 normal capacity magazines from Brownell's that were in my possession in Idaho after July 1 2013. I couldn't tell a grandfathered magazine from a contraband magazine myself and I don't plan to say I dare you to a prosecutor in Colorado - though it's tempting.