Since caulk should be used to fill the joint where the floor meets the wall (and in any change of plane, for that matter), I don't see how the wall grout could "bleed" into the floor. My guess is that they want you to stick with one color because it's faster and easier (read: cheaper) for them! Also, with the right grout/sealer, there shouldn't be anything to really scrub anyway, so pick the color you like best (I think they'd all look good!)

Thank you!