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