According to Blum, the appearance of color, like all dream elements, is shaped by contributions from the id, ego, and superego. The meaning of the color may be genetically overdetermined and complex. It is more likely to appear with ego or superego re-enforcement of voyeurism. It serves both as defense and communication, may represent internal body contents, such as blood or feces, as well as body surface, and may express identification with an object or serve in differentiating self from object. It frequently expresses affect: red for anger, green for envy, yellow for cowardice.