The full moon changes color because it is being lighted slightly by sunlight passing through the Earth's atmosphere.

In this lunar eclipse viewed from Merritt Island, Fla., the full moon changes color because it is being lighted slightly by sunlight passing through the Earth’s atmosphere. The earlier reddish color is fading but still somewhat visible on the darker, upper portion. Eclipses occur when the Sun, Earth and Moon line up. They are rare because the Moon usually passes above or below the imaginary line connecting Earth and the Sun. The Earth casts a shadow that the Moon can pass through - when it does, it is called a lunar eclipse.