How can a father get physical custody of a 5 month old in Colorado?

Since you were apparently never married to your "girlfriend", in Colorado your only option is to file a petition for allocation of parental responsibilities in the District Court of the County were you are living. Until there is a court order specifically stating what your legal rights are, including any physical time with the child, you don't have any rights and she can pretty much do what she wants. See a lawyer for a better understanding of your options based on the actual current facts.