Ironically, Colorado was one of the early States to repeal its analog to the 18th Amendment, so we've thumbed our noses at the Feds before. The Federal government cannot force Colorado law enforcement officers to enforce Federal laws, and any attempt to do so would be unprecedented and almost assuredly unconstitutional. The Feds can enforce Federal law in Colorado, but they will have to get convictions from Colorado juries. The resources required to police Federal marijuana laws using only Federal cops make it prohibitive. I think its time for some training of the public on the availability of jury nullification. I believe the District Attorneys of at least Denver and Boulder have stopped filing possession cases and made announcements to that effect. I do suspect that the Feds will attempt to take some action against open pot retailing at stores, but since the law specifically allows growing one's own cannabis and specifically allows giving it away, in the end it will be like pissing in the ocean for them to bother. I expect that we are also about to revisit the Commerce Clause.