"Do I have to file a nonresident tax form to pay the Colorado earnings for my time there, and will Colorado refund the balance of the withholding?"
Yes to both questions. Colorado can tax you as a non-resident on income you earn from work physically carried out within Colorado. Colorado will refund any excess Colorado income tax withheld from your pay.
Here's a link to the detailed info about Denver's Occupational Privilege Tax:
https://www.denvergov.org/content/dam/denvergov/Portals/571/documents/TaxGuide/TaxGuideTopic61_Occup...
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
