Apparently I'm not the only one concerned. In his blog, Frank Miller responds to fan concerns about the trailer and movie. He emphasizes that the film won't duplicate SIN CITY, the adaptation of Miller's comics miniseries that he co-directed. Despite what the trailer suggests, he states the film will be full-color (as the Spirit comics were). He explains the cinematic Spirit's change in wardrobe from a blue hat, suit and mask to basic black. (Think Superman's "blue" hair.) And he comments:
"And THE SPIRIT as some sort of SIN CITY REDUX? No, SIN CITY, that one's my own baby, folks, and it looks the way it does for its own reasons. THE SPIRIT is, and will always be, Eisner's SPIRIT. Anybody watching me on the set could attest that I very frequently drew a storyboard for a given shot first as I saw it, then as Will might’ve seen in—and, in every case, went with what I saw as Will's version."
I'll give him the benefit of the doubt.