Colorado can do (and did) as they wish. 10th Amendment to the max!
However, the hard and fast rule applies: Play stupid games - win stupid prizes.
And voters there made stupid decisions that they have to live with.
You anti-legalization folks think that somehow, upon legalization, the users just showed up. Let me clue you in on something - the voters merely legitimized something that has been going on for many, many many decades. It's a horrible thing, I know. What with having those legal stores checking your kids ID instead of them getting it from the street corner dealer who doesn't care how old they are.