What can a landlord do in Colorado when this happens ?

You can take them to court for breach of contract. You may want to try to lease the property to show the court that you are trying to mitigate your damages. Have you browsed through the information in LawInfo's Free Legal Resource Center to learn more about your issue yet? See: http://www.lawinfo.com/consumer.html and http://resources.lawinfo.com/en/index.html. You can certainly try to speak to a lawyer to determine what legal options may be available. In the meantime, you may be able to learn more on your own. Search the "Free Legal Resources" tab, or browse the Consumer Resources. Good luck.