Colorado is already a state has been for a while sometimes they think of making of republic or whatever you call a state like pueto rico
What I think sgluckadoo was talking about was that there were a few counties in Colorado (and a few neighboring states) that wanted to secede from their respective states and to form a new one (see
http://thehill.com/blogs/ballot-box/other-races/189411-6-of-11-counties-in-colorado-vote-to-secede). The problem is that these counties are more rural areas of the state and so their views become under-represented from the more populous urban areas, esp. around Denver.
When I lived in New York many years ago, we also heard of some people wanting to secede to create a new state... The main problem was that New York City has such a large population that most of the state laws and financing was geared to help the voters in that area, leaving the "Upstate" without much help from the state.
I also know that here in MN, there is an area in the most northern part of the state (known as the Northwest Angle:
http://en.wikipedia.org/wiki/File:NORTHWEST_Angle.png) that will every once in a while try to secede from the union itself. This part of the state is not connected by any US roads to the rest of the state, and people must travel through Canada in order to get to it.