Mostly moving between sky renders in Vue to PhotoShop and back at the moment...
While some of the work is about creating nice imagery of Mars, a lot of it is actually just fixing consistency problems... For example, adding clouds to the sky in one shot because it's part of sequence that was shot earlier - when it was cloudy.
Mars is actually not particularly cloudy, but I've taken a bit of artistic liberty and decided to stick with cloudy skies. This is partly because it's easier to leave the clouds in the footage already shot (and add clouds to blue-sky shots), than take them out, and also because it looks great.
It's kind of embarrassing in a way, because it's the sort of thing that the geek in me gets irritated about in other people's sci fi movies!