Calvin J. Hamilton's website View of the Solar System describes this image as follows:

"This picture Gaspra is a combination of the highest-resolution images and color information obtained by the Galileo spacecraft. The Sun is shining from the right. The subtle color variations on Gaspra's surface have been exaggerated. en:Albedo and color variations are associated with surface en:topography. The bluish areas are regions of slightly higher albedo and tend to be associated with some of the crisper craters and with ridges. The slightly reddish areas, apparently concentrated in low areas, represent regions of somewhat lower albedo. In general, such patterns can be explained in terms of greater exposure of fresher rock in the brighter bluish areas and the accumulation of some en:regolith materials in the darker reddish areas. (Courtesy USGS/NASA/JPL)" [1]