We measured reaction times for detecting the onset of motion of sinusoidal gratings of 1 c/deg, modulated in either luminance or chromatic contrast, caused to move abruptly at speeds ranging from 0.25 to 10 deg/s (0.25-10 Hz). At any given luminance or chromatic contrast, RTs varied linearly with temporal periodicity (r2 congruent with 0.97), yielding a Weber fraction of period. The value of the Weber fraction varied inversely with contrast, differently for luminance and chromatic contrast. The results were well simulated with a simple model that accumulated change in contrast over time until a critical threshold had been reached. Two crucial aspects of the model are a second-stage temporal integration mechanism, capable of accumulating information for periods of up to 2 s, and contrast gain control, different for luminance than for chromatic stimuli. The contrast response for luminance shows very low semi-saturating contrasts and high gain, similar to LGN M-cells and cells in MT; that for colour shows high semi-saturating contrasts and low gain, similar to LGN P-cells. The results suggest that motion onset for luminance and chromatic gratings are detected by different mechanisms, probably by the magno- and parvo-cellular systems.