Since you have not replied and I will be logging off shortly I will provide an answer based on some presumptions. You may want to reply and change the presumptions I used or the facts of the case as that may change my answer. If you have a valid order that has been entered by North Carolina that grants you primary possession. Different states use different language, but if the order provides that you are the primary care giver and/or you decide where they live then you would need to have your Court order taken into the Colorado court as they are required to follow that order. Follow this link to read more http://www.uccjea.net/Colorado-Act-2012.pdf UCCJEA ACT look at the following :
14-13-313. Recognition and enforcement A court of this state shall accord full faith and credit to an order issued by another state and consistent with this article that enforces a child-custody determination by a court of another state unless the order has been vacated, stayed, or modified by a court having jurisdiction to do so under a provision of law adopted by that state that is in substantial conformity with part 2 of this article.
You will find it on the pdf I sent you under index term Recognition and enforcement.
Note this will be fairly complex and it is highly recommended that you retain an attorney in Colorado to handle the portion of the case that will take place in Colorado. You may need that Court to go through the legal steps to enforce your custody agreement. The Court has within its discretion the power to award you all costs, including attorney's fees, cost of court, travel, etc. None of that is relevant if she has nothing as you won't be able to collect the money. I know this is frustrating and I mention that just so you understand that even if you get a judgment for fees you have to collect it and that can be challenging.
From your question I don't know if the mother has some rights to visit or none and whether she was authorized to take children to Colorado. Either way if she has taken them beyond allowed time then she is in violation. My best understanding is the only way you can get emergency relief is to show the children are at risk of imminent harm. If that is the case you can fairly fast action and all these things cost money, but the ACT allows the Judge to send law enforcement out to recover the children based on potential harm.
If no potential harm I believe that you will likely have to go through the steps of having her held in contempt by the Court in Colorado and then if she still refuses to turn children over the Judge would typically issue a bench warrant for her to be arrested. A local attorney can advise you better. I would think you would want to alert their version of child protective services so in the event mother is arrested they know how to reach you so they can have you take the kids vs temporary foster care, etc.
The majority of attorney's you speak to will likely give you a 30 minute consultation over the phone to outline what your rights are and the action they would contemplate taking to enforce your order. You can likely do this over the phone. It would probably be helpful once you schedule a time to talk that you send a copy of your N.C. order to the attorney so he can review it with you during the call. Due to the cost of travel do note that the attorney can do most of the leg work and you might just need to show up for a hearing.
Look for an attorney in the same County as children as that will make things a little cheaper (won't have to pay attorney just for driving to Court). I have found that avvo.com is a good place to search for attorney's. The state bar of Colorado may also refer you to an attorney, but note that usually attorneys just pay a certain sum each year and the bar sends clients to them. On Avvo you can read a little more about the attorney. For this type of matter you want to find someone that primarily only handles family law. Also ask them how many interstate enforcement actions they have handled. If the answer is 1 or 0 call a different attorney. If you can't find one that has handled it then find the attorney that is most dedicated to family law and they will simply have to research all the steps.
I hope that answers your question and provides some guidance. Thank you in advance for giving me positive feedback as that is the only way I am compensated for the time I spend answering questions. Do note that even after giving me a rating you are welcome to reply back here with follow up questions related to your original question.
Do note that I will be traveling tomorrow so I will likely not be back online until after 7:00 PM CST. Feel free to reply back with any other questions or comments and if I miss you I will respond when I get back tomorrow evening.