The Martian Surface is illuminated both by the direct solar flux and by a diffuse flux originating from scattering by dust in the Martian atmosphere. The fact that the diffuse flux varies in intensity and color across the sky leads to nonsolar total illumination of surfaces. This, in turn, can lead to interesting phenomena such as the apparent color changes of rock surfaces with time of day <Thomas et al., 1999>. A further consequence is the possibility that two surfaces, in an identical orientation and of identical color when illuminated by a direct beam, can change color relative to one another when viewed under Martian conditions if the two surfaces are of different surface roughness. This is a natural consequence of microshadowing combined with the diffuse illumination of those shadows. In this paper, we demonstrate the phenomenon with a numerical model, show an example from the Mars Pathfinder landing site where it could be of significance, and discuss its implications for spectrophotometric studies of Martian rocks. ¿ 2000 American Geophysical Union