i think the most artistic sigs are black and white, and they are often then best, however if you screw up you screw up worse, and somtimes colour gives a great effect
Imo, Black and White is better, and it gives more depth. It also helps those people who may suck at coloring sigs. I don't like color all that much, or when I do make sigs with color, I add as less of it as I can.
i think having a grayscale sig can look nice in some situations, but i think a lot of the times the artist is too lazy to put some time into color
Black and white gives more depth to sigs, whereas color just adds attractivess and flashiness to them most of the time.