Coloradans love the outdoors so even if you get away from the front range the dispersed camping spots may be filled on the weekends. It's much better on the western side of the Rockies though - less traffic and fewer people.
We've been in Colorado for two months, in the Denver area for half of it, and have been staying in Walmarts a lot because we wanted to see things around the city. Getting a campsite at most of the public parks near the city is very hard.
I've made a Google map with places where we've stayed for free, not all dispersed camping though. Some of the places west of the Rockies are very nice.
https://maps.google.com/maps/ms?msid=210711839070103512786.0004e039a19ec37a6a084&msa=0&ll=38.642618,-106.490479&spn=4.856286,8.031006