Speaking with Entertainment Weekly at New York Comic Con, Frank Miller points out the differences between The Spirit and Sin City:
It’s in color, for one thing. But also it’s a very different movie. Sin City was really a combination of me and [co-director] Robert Rodriguez working off my own drawings. And this was based on comics from the 1940s — Will Eisner’s — but through a much more modern lens. Besides, you’ll never see a tie this color [motions to his half-red, half-blue Spirit tie].
So, that should clear up everything.
Miller also tells the magazine he’s written Sin City 2, which he plans as part of a movie trilogy.