When a transparent filter is moved in front of a variegated surface, observers can see not only the surface through the transparent layer but also the layer itself. Separation of the stimulus into the underlying surface and the overlaying layer is termed color scission. Implicit is the notion that scission leads to transparent layer constancy and surface color constancy. Consider a red filter overlayed on a set of colored materials and the same red filter in front of a set of gray materials. The appearance of the local colors of the two overlaid regions will be different. The question is whether observers can extract transparent layer properties common to the two overlaid regions, and tell whether the two filters are identical.

Color perception in the "illumination" mode always accompanies the perception of object colors. The color perceived to belong to the illumination of the objects is based on clues from the scene within which the objects are perceived instead of being based on any view of the source itself. By matching spectrally filtered spotlights on colored materials with spotlights on gray materials, these clues can be ascertained.