Man and his precursors have had billions of years to adapt to Earth. And Modern Man requires so much himself and the infrastructure of a colony would require so much it wouldn't be possible.

Machines, spare parts, antibiotics, food, gravity, cloth, chemicals, glues, hospitals, hospital equipment, antiseptics, anesthesia, surgeons, schools to teach future surgeons, and so on.

And Mankind doesn't have the ability to live in close proximity to each other. The Japanese do the best, and they aren't crowded like a large Mars colony would be.

And it would have to be communistic. When survival is important, all resources have to go for the community. Can you imagine an army which had someone selling troops ammunition, and not giving those without money any bullets? Or a McDonalds on a nuclear submarine handling the food prep and distribution. One guy loses in a crap game and won't eat for another 2 months? (The idea that a person won't perform without a financial incentive is repudiated every day in the armed forces. And by volunteers. It's simply nonsense, proven, demonstrated beyond any refutation.)

So we can forget the idea we'll colonize anything successfully without vast support from Mother Earth. Won't happen. If we can't make it here we can't make it anywhere.