Colors of the Moon (as viewed from Earth)

The term 'blue moon' can mean one of two things: a fairly uncommon occurrence where a full moon emerges twice in the same month, or like the phrase suggests, the actual blue tinting of the moon. Depending on where one lives, there is a greater chance of witnessing an actual blue moon, rather than a second full moon. This is the case in large polluted cities, areas prone to forest fires, and in areas of volcanic activity. Suit and ash particles from pollution and smoke tend to deposit high into the Earth's atmosphere, and as a result can cause the moon to appear blue. The moon can take on a red hue as well - a common attribute of lunar eclipses. During an eclipse, the moon remains visible due to residual (remaining) light deflected towards it from the Earth's atmosphere. This light is primarily from the red part of the light spectrum, and thus tints the moon red, and sometimes even brown. Finally, the moon appears orange more in the Fall than during any other time of the year, because it is celebrating Halloween (credit for the bad joke sited below). The actual reason for this is the moon's trajectory path. During Fall months, the moon stays low to the horizon, and therefor its light has to travel through more atmosphere than usual. As a result, blue, green, and purple light is heavily scattered by the time it reaches our eyes, and we are left with an orange moon in the sky.

sources:
http://www.cnn.com/2004/TECH/space/07/30/blue.moon/index.html
http://dsc.discovery.com/news/afp/20031103/redmoon.html
http://www.windows.ucar.edu/tour/link=/kids_space/orangemoon.html (bad joke)