|Well, living within technological "shells" is the way that we've interacted with alien environments up to this point. To be honest that'll be the way it'll continue for the next few decades or even centuries. Without radically re-designing the human form to be able to exist unprotected on, for instance, Mars then it's the only way to go without making the planet fit the organism.|
But even then we'll have an effect on the planet as no suit is 100% waste efficient.
The thing is that it all depends on the scale of colonisation. If simply using the place as a springboard to other worlds is the goal then it'd be possible to remain inside a technological shell (i.e. a base, a suit, a rover vehicle). However, if establishing a society with a view to making it independant of Earth is the goal then some level of human adaptation to the environment is needed (even if it is for things as basic as gravity).
From the wiki on Colonisation of Mars.
There are differences, of course, between Earth and Mars:
The surface gravity on Mars is only one third that of Earth. It is not known if this level is high enough to prevent the health problems associated with weightlessness.
Mars is much colder than Earth, with a mean surface temperature of -63°C and a low of -140°C.
There are no standing bodies of liquid water on the surface of Mars.
Because Mars is farther from the Sun, the amount of solar energy reaching the upper atmosphere (the solar constant) is only about half of what reaches the Earth's upper atmosphere or the Moon's surface. However, the solar energy that reaches the surface of Mars is not impeded by a thick atmosphere like on Earth, so that solar energy at the surface of Earth or Mars is largely the same. If Mars were to be terraformed, significantly less sunlight would reach the surface.
Mars' orbit is more eccentric than Earth's, exacerbating temperature and solar constant variations.
The atmospheric pressure on Mars is too low for humans to survive without pressure suits; habitable structures on Mars will need to be constructed with pressure vessels similar to spacecraft, capable of containing a pressure between a third and a whole bar.
The Martian atmosphere consists mainly of carbon dioxide. Because of this, even with the reduced atmospheric pressure, the partial pressure of CO2 at the surface of Mars is some 52 times higher than on Earth, possibly allowing Mars to support some plant life. Most higher plants cannot survive without a minimum level of oxygen, however.
Mars has no magnetosphere to deflect solar winds.
Adapting to the planet means having to learn how to modify our physiology to breathe CO2 in a vacuum level atmosphere. Teraforming is not necessarily practical and would be a massive task with unforseen results.
(Sorry if I get a little too Lab, it's hard to talk Martian colonisation without doing so...for me anyway).