One of the theoretical questions about comics I have been thinking about lately (seriously!) is the difference color makes in comics. Even the usually reliable Scott McCloud pretty much punts on the question of color in his groundbreaking Understanding Comics, where the chapter on color is the shortest chapter in the whole book and relies heavily on a discussion of the history of color separation, four-color printing technology, and so on. Though I don't have my copy at hand, as I recall it, he ends up addressing the meaning or effect of color quite briefly and ineffectively. He shows a black and white panel (a volleyball in the air over the net) and claims it shows or implies motion, while the same panel, when colored, looks more like a static picture of a frozen moment in time.
To a degree, this comment works well enough in context: though McCloud doesn't put it quite in these terms, he suggests that the black-and-white panel is more "iconic" (in McCloud's sense) than the color, and thus encourages the reader's more active participation in the reading process, while the color panel is more representational and hence more liable to be read as external to the reader's participation. In short, the reader's participation makes the black-and-white panel seem like (subjective) action and the color panel seem more like (objective) photography.
(One could, perhaps, test such a theory: do color comics make more or less use of non-representational features such as motion lines, stink lines, and the like? Do these function to overcome the supposedly static effect of color comics? Or do they occur more frequently in black-and-white comics because black-and-white comics are inherently less representational?)
Since I am generally skeptical about McCloud's arguments about reader identification (and hence reader "participation"), I am suspicious of this line of argument. But I believe one thing is clear: in black-and-white comics, everything that is visible, everything that makes up the comic, is literally in the lines. In color comics, on the other hand, the colors serve as an additional visual element, filling in those lines (and, consequently, diminishing the visual impact of the lines). Both because the colors are often added by a different person than the artist and because of this diminution of the visual impact of the lines, I think color must minimize or diminish the effect of the comics artist's individual style (though some works, of course, may take more advantage of color from the start; see the sometimes-disorienting effects of V for Vendetta's non-use of outline drawing). Although I'm not sure how far I want to push it, I wonder if the use of color in comics doesn't serve to (implicitly) place the emphasis on those aspects of narrative that have the least to do with the artist's style: the plot, the language, the pacing. But I'm still trying to work it all out.
Regardless, it's small wonder, perhaps, that so many of the comics creators most powerfully concerned with establishing a style work in black-and-white.