It seems that the brightness (exposure, light intensity) in the material editor preview is higher than in the scene. If you put a sheet of paper in a dark room, it will be dark. And that's what happens here.
You could check if everything is fine by applying a clay material (no textures, bump maps, just diffuse color) to the same object. If it also appears darker than in the material editor, then the difference is caused by exposure and lighting.