I think most colleges will be chiefly left-leaning, owing to the fact that they're filled with students, but some are likely to be less left-leaning than others. As Melancholy said, you're likely to be happy whichever college you end up in (assuming you get in), so it's worth doing some research to find out things like accommodation types, accommodation costs, food costs, location with relation to departments/libraries, distance from the centre of Cambridge and that sort of thing.