If a colony existed on Mars, supplies and resources would have to be shuttled back and fourth from Earth to Mars on a regular basis. That being the case, there is no reason why a colonist should not be able to snag a ride on one of those shuttles to return to Earth.
It's several orders of magnitude cheaper to send unmanned one-way supply-ships then it is to send anything that can return.
You'd have to bring the fuel to return too, plus life-support and radiation-shielding to survive the couple-of-months travel time.
Plus have systems to go into orbit, have something that can launch to that orbit, etc...
Have the ship land? even more fuel, and a couple of magnitudes more expensive.
Not even the ship that would be built to take them there would be built to return, but be re-used to build the colony
I think, if and when the first humans land on Mars, they will be landing in a ship designed to return them to Earth. I don't believe that the body sending them on that mission (likely governments, not TV producers), will not task the first expedition to Mars with establishing a colony at their destination.
That means we will already have conquered the challenges of getting humans to and from Mars. Every little thing we do in a manned mission to Mars will be outrageously expensive, and even more so in building a colony.
In truth, there will likely be a space station orbiting Mars before a colony is built. The station would be equipped with Earth Return Vehicles which would carry people back to Earth. That negates having to worry about getting a larger ship intended to be inhabited for months at a time on and off the surface of the planet. It would never have to even enter the atmosphere.
Crew and supplies would be dropped from the station to the Martian surface, and it would only require a capsule capable of taking off in the thin Martian atmosphere to rendezvous with the orbital station.
NASA has some fascinating articles about colonization and travel to Mars: http://search.nasa.gov/search/search.jsp?nasaInclude=manned+mission+to+mars