The S and (M+L) cone opponent color mechanism is responsible for seeing a yellowish apparent color at the transition of black to white and bluish one at the transition of white to black when Benham’s top is spinning. To better understand this mechanism, we investigated the effects of adding blue or yellow sector at the transition edges. Experiments demonstrate that yellow sector affects percepts at both transitions of black to white and white to black, but the blue sector affect only at the transition of white to black.