Mawrth Vallis is a place on Mars that has fascinated scientists because of the clays and other hydrated minerals detected from orbit.
In this image, the enhanced black colors are most likely basaltic sands and rocks, while the green, yellow, and blue colors correspond to the different hydrated minerals.
This particular image was taken of a location in Mawrth Vallis that has a mineral called jarosite. Jarosite on Earth forms under wet, oxidizing, and acidic conditions. Another place on Mars where the Opportunity rover landed and explored also has jarosite. [More at link]