Colonizing mars is a stupid thing to do pretty much full stop.
Mars cannot be terraformed. It has no meaningful magnetic field. That's a dead end for a planet's ability to retain a useful atmosphere and ward off cosmic radiation. You'll have to live in shielded, sealed pods to survive there, always. In which case you may as well be in space with access to abundant solar energy and less of a gravity well to contend with for your frequent resupply missions. There's barely any advantage to being on Mars compared to orbiting some random Lagrange point or the sun itself. I guess you can make use of some earthworks for civil engineering? Hardly seems worth it.
There's also no point in colonizing mars. We have enough uninhabitated desert here on earth. It's not a space problem, it's a cost problem. And building on mars is a lot more costly than building in a desert.
I've read that Mars could retain an Earth-like atmosphere because, while it's stripped away by the solar wind, it would happen over tens of millions of years; any remotely plausible terraforming attempt would be able to replenish it much faster than that.