Color choice is a fairly personal thing, so consult the person in the mirror. Black is fairly standard and boring for cabs. If you decide you don't like the first color you pick, paint over it until you find one you like. Hot pink, fluorescent orange, fire-truck green, bright yellow spring to mind as "making a statement that the cabs are not standard and boring" but if dark blue floats your boat for that job, that's what you should use. Do be slightly wary of the way that a small color swatch is different than a full-scale colored object.
Then there's stripes, spots, geometric patterns, patterns borrowed from animals, etc. as opposed to just solid color. Or white with 5 black lines, an F clef, and some notes.