Seeing the Invisible Colors of Mars

The most important thing about the colors on Mars is that they are responsible for letting us know what minerals are present on the planet, and therefore can give us a clue about the past or current presence of water. The red to orange color that everyone thinks of when they hear about Mars is known to come from iron oxides. What a spectrometer can see that our eyes cannot are more colors that represent the many other mineral compositions that have a certain spectrum. More technically, spectroscopy is measuring atomic bonds in the crystal structures of each mineral. In transferring spectrometers from Earth to Mars, scientists must remember to set their equipment to measure a different wavelength range and resolution than on Earth.

http://www.space.com/searchforlife/seti_mars_color_040129.html