When we look at the other planets we see different colors because we are looking at different surfaces. When we look at VENUS, we are looking at the top of the atmosphere of Venus. Venus is permanently shrouded in clouds and when sun light falls on the clouds from above, the clouds REFLECT the light. Hence Venus appears white and BRIGHT.

When we look at MARS we are actually seeing the rocky surface of mars. Now Mars has a thin coating of RUST (iron oxide called the mineral hematite, Fe₂O₃) that coats the surface. Just like rust on a piece of metal left outside on Earth. Hence when we look at Mars we see RED, the color of RUST.

When we look at the Moon we are seeing light that is reflected off the surface rocks. These rocks are of two types: the highland area is rock type called anorthosite...mainly feldspar which reflects light and is whitish. Then there is another rock called basalt that is DARK.

Go out tonight and look at the moon; can you see the two different types of rocks based on the brightness?

Different planets are made of different kinds of matter, and as such, they reflect different frequencies of light, thus having different colors, for the same reason anything else have different colors. In the case of the planets, these colors are determined to a large extent by what kind of compounds can exist on them, and that is determined by their temperatures.

The planets appear as having different colors because of the things on their surface. For example, Mercury appears grey because it is made up of rocks of the same color. Venus is yellowish-white because of the sulfuric acid clouds. Mars is red and orange based on the rusty rocks on its surface. Jupiter and Saturn have ammonia clouds which leads to a white or pale yellow. Uranus appears light blue due to methane clouds.

NASA has recently launched the Cassini spacecraft with some beautiful images of Saturn and its biggest moon called Titan. Check those out!