Do not adjust your monitor: Mars hasn't really been given a rainbow paint job. In fact, this image is a color-coded topography showing the heights and depths of the planet's Ismeniae Fossae region.
In the image, white and red show the highest terrains, while blue and purple show the deepest. Imaged by the High Resolution Stereo Camera on ESA's Mars Express , you can see the 2 kilometer-wide valley carved out by ancient glaciers and ice-rich flows. If you're wondering what all the virtually perfect circles are, the ESA explains :
In the left portion of the scene the channel truncates a roughly 25 km-wide crater. Material in the crater walls has slumped down into the channel, smoothing over the grooved floor. Around this crater, and elsewhere in Ismeniae Fossae, clusters of circular to elliptical, partially interconnected depressions are observed. These may be either secondary impact craters from debris flung out by larger impact craters, or collapse pits caused by the sublimation of subsurface ice.
And, just in case rainbow colors are a little wild for your tastes, you can see the true-color image below. See, Mars is still red. [ESA]