|MadSci Network: Astronomy|
Photos of Mars from both spacecraft and telescopes do indeed reveal different colored regions on Mars. Generally, these regions have simply been referred to as "light" and "dark." The dark regions were observed through telescopes years and years ago to grow and shrink with seasonal variations. This initially led astronomers to speculate that the dark regions were composed of vegatation. We now know that these variations are due to seasonal dust storms blowing different colored dust across the surface of Mars.
There are indeed regions which are consistently light or dark colored on Mars. Their colors are due to the chemical composition of the land. Certain minerals in rocks and sand will either reflect lots or little of light making their appearance either light or dark, respectively. As I am not a geologist or planetary scientist, I cannot tell you exactly what minerals are currently thought to primarily make up the light and dark regions on Mars. But I can direct you to a nice press release from the Space Telescope Science Institute. This press release describes some of the minerals suspected of causing the light and dark regions seen in some Hubble Space Telescope images taken of Mars in 1997.
I hope that helps answer your question.
Try the links in the MadSci Library for more information on Astronomy.