A page from the "Causes of Color" exhibit...
Why does Mars appear red and Earth blue-green?
Planets are an exception in the night sky, because we see them through the light they reflect – they are not sources of light. However, they do reveal remarkable colors in the light they reflect.
The variety of color in planets depends on a wide range of factors. If they have an atmosphere, the color we see may be related to weather effects in their atmosphere, such as the clouds on Venus, or to the gases themselves, on the gas giant planets of Jupiter, Uranus, Neptune, and Saturn. Earth looks blue-green from space, depending on cloud cover, because of the high proportion of water on the surface.
Planets with little or no atmosphere are seen in the color of their surfaces. Mercury is a rocky grey, while Mars is a butterscotch, reddish color (have a look at the Martian sky).