  Other observed effects, however, require a theory whereby the same atoms can produce different effects without supposing that the observer has changed.  The change must then occur in the object seen. The explanation of color seems to be of this variety: Aristotle reports that things acquire their color by ‘turning,’ tropê (GC 1.2, 315b34).  This is the Democritean term that Aristotle had translated as ‘position,’ thesis, i.e. one of the three fundamental ways in which atoms can appear differently to us. Aristotle gives this as the reason why color is not ascribed to the atoms themselves.  Lucretius' account of why color cannot belong to atoms may help clarify the point here.  We are told that if the sea's atoms were really blue, they could not undergo some change and look white (DRN 2.774–5), as when we observe the sea's surface changing from blue to white.  This seems to assume that, while an appearance of a property P can be produced by something that is neither P nor not-P, nonetheless something P cannot appear not-P. Since atoms do not change their intrinsic properties, it seems that change in a relational property, such as the relative position of atoms, is most likely to be the cause of differing perceptions.  In the shifting surface of the sea or the flutter of the pigeon with its irridescent neck, it is evident that the parts of the object are moving and shifting in their positional relations.