Whenever the colony is shown, we see it is raining there. What is the reason for that? I believe there must be a reason behind shooting the scenes in rain?
Quoting Colin Farrell:
The film’s split into two hemispheres – one being the Colony and the other being the United Federation Of Britain.
So over in the Colony, which is mainly full of proletariat, the weather’s more symbolic of the more downtrodden existence that the people there have. So it’s always dark and rainy.
And then in the UFB, which is very unlike Britain, the skies are sunny and clear and things are a little bit more optimistic.
Any chemical-warfare disaster that could kill off the majority of earth's population could easily have included nuclear bombs or some other disaster that could affect the weather. Or, whatever efforts are in effect to keep those areas clear involves a lot of rain.