Are Colorado citizens required to register their firearms? NO. In fact, state law prohibits firearms registration by local or state government (the federal Brady Act is another matter, though that's not "claimed" to be registration).
CRS 29-11.7-103. Regulation - type of firearm - prohibited.
A local government may not enact an ordinance, regulation, or other law that prohibits the sale, purchase, or possession of a firearm that a person may lawfully sell, purchase, or possess under state or federal law. Any such ordinance, regulation, or other law enacted by a local government prior to March 18, 2003, is void and unenforceable.
CRS 29-11.7-102. Firearms database - prohibited.
(1) A local government, including a law enforcement agency, shall not maintain a list or other form of record or database of:
(a) Persons who purchase or exchange firearms or who leave firearms for repair or sale on consignment;
(b) Persons who transfer firearms, unless the persons are federally licensed firearms dealers;
(c) The descriptions, including serial numbers, of firearms purchased, transferred, exchanged, or left for repair or sale on consignment.
What is required when I bring firearms from another state? NOTHING
Is it legal for a private citizen to sell a firearm to another private citizen? YES, as long as the seller has no knowledge that the purchaser is ineligible to purchase that firearm. Click here to sell firearms privately.
Is it legal for a private citizen to sell ammunition to another private citizen? YES.
Does Colorado require "handgun registration"? NO (Only because of the hard work of RMGO and it's members).