How old is the car ? Car color do change over time specially if it is parked out in sun. That could be reason of touch up not matching with car color now.
I think this is the answer. Touching up metallic paint doesn't work as well.Did you make sure that the touch up paint was thoroughly mixed before application? Have found the MB touch up paint pretty good match wise, but Metallics are always difficult to get a good colour match due to brushing rather than spraying.