There is no such thing as legal medical marijuana. The repealing of a state statute doesn't make something "legal" when it is still prohibited by federal statute, as marijuana is. It's simply a federal offense that this administration chooses not to prosecute vigorously.
The part about the federal law is still true but as far as I know, law enforcement in Colorado is not required nor can be forced to enforce federal law.
But the FBI can very well decide to prosecute federal crimes in states, even if the "crime" is legal under state law.
We have a Pay-To-Play legal system. Vince Neil of Motley Crue killed a guy with his car, paid a fine, pled guilty, and served two days of jail total. He's just one example.
(https://www.vice.com/read/you-can-kill-anyone-you-want-with-... and http://www.nytimes.com/2013/11/10/opinion/sunday/is-it-ok-to...)
However, it's easily dischargeable. This is conjecture, but I wouldn't be surprised if many of those cases were settled privately.