Can you attend a separate college to get a minor?
Technically speaking, no.
If you are not enrolled in a degree program at an institution, you technically cannot earn a "Minor" in a discipline.
However, you can still take courses at another institution, and you can work towards a non-degree certificate. Oftentimes, universities and colleges may offer continuing/professional education courses that provide a certificate in a specific discipline. Certificate programs tend to amount to the same amount of credits as a "minor."
An Associate's Degree at another institution may also be an option.