The color is based on the visible to near-infrared spectrum, like many Mars Reconnaissance Orbiter HiRISE images; color differences are more exaggerated than is real. You can see that the area on the left -- part of the ejecta of the Stickney crater, out of view here -- is grayer than the rest, which is yellowish-brownish. (You can also see some color fringing, where the color channels don't align perfectly; that's a result of the way that CaSSIS color imaging works, viewing Phobos from slightly different directions for each color channel.) It's common for crater ejecta to be grayer or bluer than surrounding terrain, because planetary surfaces get browner with longer exposure to space weathering, and more recent impacts expose unweathered material.

It's a neat pic on its own, but neater when I realized that we don't commonly see Phobos from this angle. Most Mars orbiters operate in orbits that are much closer to Mars than Phobos orbits. Since, like our Moon, Phobos always keeps the same face pointed at Mars, that means most Mars orbiters most commonly only look at its Mars-facing hemisphere. Of recent missions, the one that has done the most Phobos imaging is Mars Express; Mars Express has a highly elliptical orbit that enjoys regular seasons of Phobos flybys. A long time ago (eight years ago, yikes), I took a thorough look at how well Mars Express had covered Phobos, and the answer was: pretty well -- except for the trailing hemisphere. Here's the summary image from that post: