Most empirical work on color constancy is based on simple laboratory models of natural viewing conditions. These typically consist of spots seen against uniform backgrounds or computer simulations of flat surfaces seen under spatially uniform illumination. We report measurements made under more natural viewing conditions. The experiments were conducted in a room where the illumination was under computer control. Observers used a projection colorimeter to set asymmetric color matches across a spatial illumination gradient. Observers' matches can be described by either of two simple models. One model posits gain control in one-specific pathways. This diagonal model may be linked to ideas about the action of early visual mechanisms. The other model posits that the observer estimates and corrects for changes in illumination but does so imperfectly. This equivalent illuminant model provides a link between human performance and computational models of color constancy.