If you are asking about totally intrastate manufacture, sale and use, then yes, Colorado should be able to legalize Heroin and Meth.
I am not saying that they should legalize Heroin or Meth. I am not even saying they should legalize marijuana. Personally, I am opposed to all three.
What I am saying is that Colorado and every other state should have the right to legalize, regulate or prohibit the intrastate manufacture, sale or use of any drug, food or other ingestible substance as the majority of their citizens see fit. If someone in Colorado does not like the laws that Colorado chooses to enact over intrastate commerce, then they are free to go to another state with different laws. When the feds control everything, then that right is lost.
It is called federalism. All laws and regulations should be enacted at the most local level possible, so that citizens have a say in how their rights are restricted.
How about counterfeiting just so long as the fake dollars stay in Colorado?
I don't think you are grasping the notion that the consequences of some things won't remain within a state.
Now let me point out this. At some point in our history, pot usage was much lower than it is now.
Let us even speculate that at some point it was only a few thousand people using this drug. How many do you think there are out there using it now?