Landsat satellite data of will provide realistic colors and texture for the polygons, with a resolution of 30m.
Data first has to be preprocessed to correct for distortion caused by for varying elevations, using the DEM data, so that it becomes like a 2D map. The satellite image data can then be ``draped'' over the DEM data to make a 3D image.
Satellite image data at better than 1km resolution is available for Mars and Venus from JPL.