Efflorescence is caused when soluble salts and other water dispersible materials comes to the surface of concrete and bricks.   Here in Las Vegas the watering of our landscape is the daily source of moisture which starts the process.  It can occur very soon after exposure to moist or cool conditions or gradually, especially when it comes from within the concrete or from the subgrade.  Efflorescence is normally white and shows up more on darker colors than white or light gray because of the contrast. Only 0.2 ounce of calcium carbonate per square yard of surface is needed to cause a significant shift in color. Some forms of Efflorescence are very difficult (if not impossible) to remove, while if removed soon after they form they are manageable.