First of all - every contract of carriage I've ever seen says it is the responsibility of the passenger to have the proper paperwork and it is not the airline's responsibility to provide return transportation of you are denied entry to a country.
I know this is an old thread but may be useful to others in 2016.
I have traveled extensively to Colombia (maybe 12 trips since 2013) and I have never been asked about proof of onward travel or proof of finances by immigration authorities. I am a citizen of the U.S. and full time traveler working remotely, so I am almost always on a round trip ticket originating/returning to Colombia or a one way trip to Colombia. If you are ever asked just say you'll be traveling overland to Ecuador. It's possible but almost unheard of to book these buses in advance. They know this, a lot of people travel this way, so you shouldn't have a problem.
The airline may give you trouble departing the U.S. They want to see proof of return to the USA not proof of onward travel. I know it makes no sense. I'd say 50/50 chances in my experience (Jet Blue is the worst). I offer four possible solutions:
1) Make a "fake" return ticket confirmation from a different airline using real flight numbers and times. I used an old itinerary for a travel agent - text based. (Word of caution - one overzealous Jet Blue employee actually looked up the 6 letter confirmation code - for a different airline!)
2) Buy a fully refundable return ticket and ask for your refund as soon as you arrive in Colombia.
3) New laws give you 24 hours after you purchase a ticket to cancel it without penalty. With careful timing and a smartphone/airport wifi you can use this rule to get around the return ticket. I in case I have connectivity troubles in the airport I always have a trusted person who can cancel for me if necessary.
4) If you are on a long term trip, like a backpacker or something, look for the cheapest ticket returning to the U.S. that you can find, even if you have no intention of using it. I did this with a Caracus-Miami flight and it served me well for 12 months.