As always, depends on the specialty.
For example, in a typical adult subspecialty - say GI - where even smaller towns (50k in population or less) have enough patients to support more than one practitioner, then you'll be able to head most anywhere you want if you're flexible. In your Colorado example, there are (according to Wikipedia) 19 cities in the state that have more than 50k in population, so there are quite a few options. So while a plurality of residents stay close, movement is possible if you're flexible.
But in some fields where the number of positions are more limited, you may actually find a paradoxical ability to move to where you WANT. By this I mean that a lot greater percentage of people end up moving away but not always to places they anticipated because the job opportunities are more geographically limited. For example, in many pediatric subspecialties, there are far fewer jobs and they are concentrated almost exclusively at academic medical centers. So you may want to move to Colorado, but the only place in the entire state with positions is in Denver with the University of Colorado. Thus your ability to get hired is dependent solely on the timing of available positions - some years they may hire 2 people, other years none. The end result is that probably far more than 50% of graduating fellows move away from where they did their training, but of those that moved, very few actually ended up in the area that they imagined they would at the beginning of their training. In my fellowship class of 5 (PICU) none of us stayed in the same state where we trained, and only 1 of us ended up in a location we anticipated at the beginning of fellowship, and that one guy who got his dream job only got lucky on the timing. A different year and his group wouldn't have had a position to offer him. I do think that there is a lot more movement of mid-career physicians in pediatric subspecialties than in a lot of other fields because of that initial movement for the first job, and so people are constantly looking for opportunities to move to their desired location on down the line. But the point about being limited to certain cities within a particular state remains.