The color appearance of a stimulus depends on the context in which it is viewed. Well-known examples of context effects include simultaneous color contrast and color constancy. In simultaneous contrast, the color immediately surrounding a test influences its appearance. In color constancy, the visual system adapts to the ambient illumination to keep surface color approximately constant. Color constancy is a context effect because the way that the visual system interprets the light reaching the eye depends on the context defined by the ambient illumination.To predict accurately the color appearance of stimuli rendered on a CRT monitor, we need a theory of color context effects that applies to such stimuli. Simultaneous contrast suggests that the color perceived at a particular monitor location may vary with the immediate context provided by the image at other locations on the monitor (the monitor context). Color constancy suggests that the color appearance of stimuli displayed on a monitor may vary with the ambient illumination of the room in which the monitor is viewed (the illumination context), at least if images from a monitor are processed by the visual system in the same way as reflective surfaces.In this paper, we report initial experiments designed to measure psychophysically how the monitor context and the illumination context influence the appearance of stimuli presented on CRT color monitors. Our emphasis, however, is on the effect of the illumination context.