The daymarks were terribly important; however, most people tend to think of lighthouses as aides to nighttime navigation. The white would not matter as much at night since mariners would be looking for the signal from the lantern. During the day, the marks were critical to distinguish one from another. As Dave points out, in places such as NC where you have Hatteras, Currituck and Bodie in the same general vicinity, the daymarks were a vital factor. Many lighthouses which had distinguishing marks were painted over in more recent times.