Who is going to ask for your onward ticket? Copa? They sell one way tickets all the time. If you need to present one to immigration officers in Panama, you would show them your return flight on Spirit.

It really isn't Copa's business how you are getting home if you don't buy a round trip ticket. But if it is a concern, you can always buy a fully refundable one-way return ticket from Colombia and then cancel it after you've checked in.