The future State of Denver can enact all of the gun control it wants after the secession takes place. I don’t think the rest of Colorado will pay much attention.
It is possible for law abiding citizens in LA, Denver etc. to find residences in parts of their states that support the Constitution. As long as an escape to safety is reasonably available, I can't see violent upheaval. However, when the Utopians decide to enforce their tyrannical laws outside of their power bases in the urban filth, the stage will be set for real trouble.