When the monument was under construction in 1854, the Washington National Monument Society ran out of money and the project ground to a halt. Twenty-five years later, the U.S. Government took over and completed the upper two-thirds of the structure by 1884 using marble from a different quarry. The two sections closely resembled each other at first, but time, wind, rain, and erosion have caused the marble sections to weather differently, thereby producing the difference in color. A third type of marble is also visible at the dividing line between the two main phases of construction.
(my camera obviously had some dirt in it.)