In Nebraska, the age of majority is nineteen, but in terms of the Juvenile code, "Juvenile means any person under the age of eighteen". See Nebraska Statutes, Sec. 43-245(2), (11). Consistent with that latter definition, a juvenile court in Nebraska has no jurisdiction over a minor who is alleged to have committed a status offense (such as running away) after the age of 18. (If the person was already under the jurisdiction of the juvenile court for a different reason, jurisdiction may continue with the court's being permitted to restrict the minor's place of residence, pursuant to the prior court proceeding -- but in the absence of such a prior, continuing juvenile court case, the juvenile court would be without jurisdiction.)
Similarly, contributing to the delinquency of a child under the criminal code defines a child as "under eighteen years of age". Neb. Stat. Sec. 28-709. The custodial interference statute similarly applies only in relation to the custody of a minor "under the age of eighteen years". Neb. Stat. Sec. 28-316.
I'll go a step beyond Taxing Matters, and state that I see no chance of Colorado forcing a person whom that state regards as an adult to return to the custody of a parent in a different state or country; but more than that, I'm not seeing any basis in Nebraska law to compel an 18-year-old to live with a parent.
Given that the police were involved, and particularly given the number that were dispatched, I would want to know the full facts and exactly what it is that the parents alleged before suggesting that the 18-year-old is not somehow restricted from moving out. Sometimes, for reasons that may not be known or fully known to a dating partner, a young adult will remain under parental control past the age of majority -- for example, due to a developmental disability or mental illness. Also, I can't rule out the possibility that the young woman at issue remains on juvenile court probation from an incident not known to the OP, or is otherwise subject to the jurisdiction of the juvenile court.

