A company that has not been authorized to operate in Colorado is known as:

A company that has not been authorized to operate in Colorado is referred to as a nonadmitted insurer. This designation indicates that the insurer does not have a certificate of authority from the state and, as such, is not permitted to write insurance policies within Colorado. Nonadmitted insurers may still provide coverage but typically do so in a manner that is regulated differently from admitted insurers. Policyholders utilizing nonadmitted insurers often have limited protections compared to those who are insured by admitted companies, such as access to state guarantee funds in the event of the insurer's insolvency.

The other options do not accurately describe a company that lacks authorization in Colorado. For example, an undesirable carrier implies a carrier that is considered poor in quality or financial stability but does not specifically denote lack of authorization. A prohibited entity suggests a broader legal restriction, not limited to insurance authorization. An alien company refers to a provider incorporated outside of the United States but is not indicative of whether the entity is authorized or unauthorized in Colorado. Thus, the most precise term for an unauthorized insurer in this context is indeed the nonadmitted insurer.