It is a theory proposing that an emotional event elicits two (2) competing processes:
(1) an a-process or primary process directly elicited by the event, and
(2) a b-process or opponent process that is elicited by the a-process and serves to counteract the a-process.
Moreover, Opponent-process theory is a Theory of color vision based on three (3) coding systems (red or green, yellow or blue, black or white); a theory that humans perceive color in terms of paired opposites: white versus black, red versus green, and blue versus yellow.
List of books: Opponent-process,theory