 An initial problem is “the problem of multiple realizations”: there is a wide range of different types of bodies that have colors—light sources, illuminants, surfaces (e.g., of apples, cars, cloths, paintings, …), volumes (e.g., wine, glass, atmospheres, …), bodies that scatter light, bodies that diffract light, films, and luminescent bodies. The causes of the colors objects appear to have, are many and varied. For most theorists, however, the most plausible physicalist candidates for the colors are light-related properties, e.g., capacities to emit, reflect, absorb, transmit or scatter light to varying degrees. For physical surfaces, the color is taken to be related to the object’s reflectance profile, i.e., the capacity to differentially reflect wavelengths from different regions of the incident illumination. It turns out, however, that, for each surface color, there is no single reflectance curve associated with that color, but many. The situation is similar, in the case of film colors or aperture colors. That is to say, for each color, there is a set of metamers. (Two stimuli—bodies, sources of light, etc.—that differ in their physical characteristics, but are matched in appearance under a certain illumination, by the same observer, are metamers for that observer, in that illumination. Two bodies that are metamers in one illumination need not be metamers under a different illumination, or for a different observer.) 