This is a semi-false color image as explained on NASA’s site, “The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter took this image of the larger of Mars' two moons, Phobos… on March 23, 2008. Taken from a distance of about 6,800 kilometers (about 4,200 miles). It is presented in color by combining data from the camera's blue-green, red, and near-infrared channels”.
So there is some exaggeration of color here by including the infrared. I am guessing that is what is making the contrast between the reddish hues and those whitish marking at the edge of Stickney Crater. Click to see the hi-res… this may be the sharpest most detailed of Phobos I have seen yet.