Yes under Federal law via ATF FAQ
https://www.atf.gov/questions-and-an...gun-unlicensed
Sell no, as a gift a limited yes under Colorado satutes
http://www.lexisnexis.com/hottopics/colorado/
I can't link directly to it so here is a copy/paste
Colorado Revised Statutes 18-12-112. Private firearms transfers - background check required
(6) The provisions of this section do not apply to:
(b) A transfer that is a bona fide gift or loan between immediate family members, which are limited to spouses, parents, children, siblings, grandparents, grandchildren, nieces, nephews, first cousins, aunts, and uncles;