All right, thanks so much for the clarification. And I know how hard it is to do things when you're out of state.
What I suggest is that you start with a dated, signed, certified/return receipt letter to the contractor, listing what is wrong, and giving him a deadline for completing the additional work. Keep copies of everything, of course.
If that doesn't do the trick, and it probably won't, the next step is to contact the state licensing agency that licenses CO contractors. You can file a complaint with them. I'd start here: http://www.colorado.gov/oed/industry-license/32IndDetail.html
If you're unable to file a complaint with that office for some reason, you can always file one here: http://www.coloradoattorneygeneral.gov/
No matter which of these state agencies you file with, you can now provide a copy of your certified letter, which will demonstrate that you made a good faith effort to resolve this matter before filing the complaint.
One of these agencies should be able to help you, and the cost will be minimal (probably just phone calls and postage).
But, if not, there's still Small Claims Court, depending on how much is at issue. http://www.courts.state.co.us/Forms/Forms_List.cfm/Form_Type_ID/9 It sounds as though your case is small enough that you can use SCC.
If you file in SCC (or any other court), be sure to ask to be compensated for the cost of filing, service of process, and interest. Ask to "appear" by telephone if you can't fly to Colorado to appear in court; most courts allow this if you ask in advance.
You asked many questions, so please let me know if you have any more questions for me today.
Thanks so much,