The human eye is far more sensitive to intensity variations than to color variations. Since CCD cameras typically vary in their sensitivity across the spectrum, it is not unusual to have excessive noise in one of the filter bands. This results in color speckling. Since the human eye effectively has less resolution in color than in luminance, it is possible to mask the color variations.
The Color Smoothing command allows you to apply a filter to the color only. It works by extracting the luminance information, filtering the three color planes, and then recombining the filtered color planes with the unfiltered luminance plane. This is a handy method for quickly cleaning up an image with color noise.