When a car is mopped (if its has lacquer ) a small amount of lacquer is removed... or rubbed off... unless you cut through the lacquer there should be no paint colour on the mop.
Not all car paint is lacquered... and on that paint you will get colour on the mop as your rubbing off colour.
Not all car paint is lacquered... and on that paint you will get colour on the mop as your rubbing off colour.