In my opinion, any attempt to colonise Mars would require a sustainable atmosphere. Without a magnetic field; This atmosphere will quickly be stripped away by solar winds. Now if we were to "Build" a large enough moon orbiting Mars by towing asteroids into Martian orbit and allowing accretion to form a moon large enough to cause tidal effects that would increase the core temperature of Mars then will it not be possible to "wake" not only the "dynamo" effect but volcanism as well. This will result in conditions more suitable for long term colonisation. I know that it will require a hell of a lot of mass in asteroids but Rome was not built in a day neither.