Theories of human color constancy have ben based on experiments with relatively simple laboratory stimuli. Even recent 'nearly natural' stimuli are optically much simpler than natural visual environments. I review here some of the complexity of natural visual environments. I argue that several kinds of optical structure exploited by theories of human color constancy may not occur in most natural scenes. Continued progress in color constancy research will require better descriptions of natural visual environments and of human color constancy performance within them. Both pose large challenges.