Space

I dont think it'll ever happen. Mars doesnt have a magnetosphere and as a result solar winds would erode away the added gasses and water vapor we manage to add into the system. Even if we were able to manage to get a breathable replenishing atmosphere I imagine the solar radiation would do bad things to us. We'd need like science fiction levels of shielding to keep it livable and we'd need to reach far off sci-fi levels of technology.

I imagine since there is an atmosphere that a pressurized dome city would be less fragile than say a structure on the moon since little holes and leaks wouldnt lead to dramatic explosive decompression. So colonization might be pretty doable but I think a green mars is unlikely.