We examine two hypotheses about the functional segregation of color and motion perception, using a motion nulling task. The most common interpretation of functional segregation, that motion perception depends only on one of the three dimensions of color, is rejected. We propose and test an alternative formulation of functional segregation: that motion perception depends on a univariate motion signal driven by all three color dimensions, and that the motion signal is determined by the product of the stimulus contrast and a term that depends only on the relative cone excitations. Two predictions of this model are confirmed. First, motion nulling is transitive: when two stimuli null a third they also null another. Second, motion nulling is homogeneous: if two stimuli null one another, they continue to null one another when their contrasts are scaled equally. We describe how to apply our formulation of functional segregation to other behavioral and physiological measurements.