While the "Next Generation Filter" is pretty annoying in and of itself, you can't really just introduce color in urban warfare. Open combat tends to turn things into rubble, plants aren't likely to survive, and the color pallet is going to be whatever color the buildings are, since that's what all the crap spewn about came from when said buildings started getting blown the fuck up. Killzone's graphics are realistic, it's just that realism isn't necessarily exciting. Far Cry and Crysis were specifically set on tropical islands because Crytek wanted an excuse to do something new after the genre had spent so long in cityscapes. The way to fix this isn't really to add more color to any given scene (there's no place to put it,) but to write the story as such that the player isn't stuck in a cityscape for the entire time.
Uncharted 2 is a good example of how you can get varied scenery, since you start in jungles, move to a city (that has a lot more color than Killzone because it's not a warzone,) move to another city that is a warzone and indeed has very little color. It's still more than Killzone, since the designers went all-out with making their environments detailed and you pass through homes with personal effects, places where water is spilling out, an expensive hotel, plus there are varieties of cars wrecked everywhere instead of the same one copy-pasted, ruined street signs, etc. After that it's up the mountains and into other places, but the city under attack, despite being "better" than most examples of "Real Is Brown," is still the least colorful area of the game.