I am not a Colorado member, law enforcement, or attorney, but I have my resources*: In Colorado it is illegal to make, convert, buy, sell, or transfer a magazine with or to a capacity of more than 15 rounds. However, it is not illegal to possess.
*Not to be taken as legal advice, but a research starting point.
My best advice for reliable information on this, outside of the Colorado Attorney General, is Michael Bane.