Well, the best tool will depend almost entirely on what effect(s) you actually want to add.

From the stills I've seen Sin City was all done on a green screen and then colourised. is this what you want to do?

And again it might depend on you NLE of choice also. For example if you edit in Premiere then export as a flm file and import that into a photo editing package like Photoshop and.or PSP.

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