There are a couple reasons why the moon appears reddish at times. One common one is because of the different wavelengths of light photons that is emmitted through earths atmosphere causing this effect. In other words shorter wavelengths usually cause the moon to glow with more of a reddish hue, particularly when it is first rising. And than, as the moon appears higher in the sky, you tend to see it turn to a more yellowish, or white hue. This is mainly because the dispersion of light at that point in time, is emmitting a longer wavelength of photons through the atmosphere.
Second most common reason that the moon appears to have a reddish tint to it, is during a total, or partial eclipse of the moon. You will notice during an evening when the moon is totally, or partially covered by the earths shadow, that it will emit a reddish hue as well.