Not sure about what the city of Denver requires. But I'm down here in the Springs so I've looked up the laws a few times. No permits are required by the state. The only things you may end up needing a permit for at all is collecting various native species here in Colorado. But even then, most of the snakes people are generally interested in do not require any permits as you can have 4 of any of those species and up to 12 total.
This is to chapter 11 of the Colorado Division of Wildlife Regulations. It states - "Unregulated wildlife may be imported, sold, bartered, traded, transferred, possessed, propagated and transported in Colorado provided that all importation, disease requirements and any other state, local or federal requirements are met. Statutory restrictions still apply" As ball pythons are not native to CO, they fall under the unregulated wildlife here. I mean, how do they regulate what's not here? LoL But yea. And it also states "No license is required for wildlife taken in accordance with commission regulation #1000(A)(6)." For regulation #1000(A)(6) see the next one.
wildlife.state.co.us/NR/rdonlyres/BDD5AB1C-D572-4458-AA3C-F7CF3BCA00AE/0/Ch11.pdf
This is to chapter 10 of the Colorado Division of Wildlife Regulations. Check out Article 1, Section A, Paragraph 6. It states "Except as provided in #015.A. of these regulations, up to four individuals of each of the following species and/or subspecies of reptiles and amphibians may be taken annually and held in captivity, provided that no more than twelve in the aggregate may be possessed at any time" and then goes on to list the species. This includes the garters, gophersnake, whiptails, racers, plains hognose and various other species of reptiles and amphibians.
wildlife.state.co.us/NR/rdonlyres/D9BADA66-8CBC-46DD-BE78-97C60EB1B813/0/Ch10.pdf
Hope that helps ya out!