Why is the moon yellow, white, red or grey?

When observing the moon from different periods of the day or year, the moon can appear to be different shades of red, yellow, or white.  The effect Earth's atmosphere has on the colors we observe is the primary factor in the moon's appearance.  The atmosphere absorbs some light and scatters mostly blue light.  This results in more of the blue light being scattered from the overall light, causing the observed light to be less blue.  If light is passing through more atmosphere to get to the observer less blue light is seen and more red light is seen.  Harvest moons occur during the autumnal equinox.  Traditionally harvest moon allowed farmers more hours of farming since the period of time between sunset and moon rise was relatively short. During the autumnal equinox, the moon rises earlier in the night and remains at a low angle with the horizon.  The light reflected by the moon passes through a greater amount of the Earth's atmosphere and gives the moon a reddish yellow shade.  This is also the reason for the sun appearing red along the horizon during sunset or sunrise.  When the moon reaches our zenith, it has a whitish grey color.

source: universetoday, wikipedia