The color is from infrared, red, and blue-green channels on the camera, so it represents light shifted slightly longer in wavelength than human eues can see, which emphasizes subtle colorations on the moon. The color view shows that the material surrounding the giant crater Stickney (on the left side of the moon) appears gray while the rest of the moon appears reddish. The grayer material is likely fresher material. From this vantage point Phobos appears about 21 kilometers (13 miles) in diameter. (The irregularly-shaped moon has a minimum diameter of about 19 kilometers and a maxiumum diameter of about 27 kilometers.)
The image has been rotated so that north is up. The view is of Phobos nearside the side that always faces Mars. This is the only face of Phobos that Mars Reconnaissance Orbiter can ever see, because it orbits Mars far below the orbit of Phobos.