Medical marijuana patients can own guns. Buying guns is different from owing guns, just like it's different from being intoxicated while possessing guns. While it's true the Feds would add charges of gun possession, they would also bust you for the weed, but how often do the feds bust patients? Local police don't charge you for possessing a gun while possessing marijuana. It's legal to possess small amounts of marijuana, and it's legal to possess certain guns. Colorado cops can't add gun possession charges to a weed possession charge because they can't charge you with weed possession.
Now, if you have more weed than allowed, then they can charge you for that, and add gun charges. But if you're not breaking the law to begin with, they have no way of adding gun possession charges. Local cops enforce Colorado law, that's why they don't bust you for small amounts of weed, or possessing a gun with small amounts of weed.
P.S. A registry card or a doctors recommendation is not proof you use marijuana, it's simply an authorization to possess marijuana for medicinal reasons. A prescription for Oxycontin doesn't mean you use it either, it means it's not illegal for you to possess a small amount of it.