In humans subliminal perception is more evident in vision than other senses (Dixon, 1987) but it has not been reported in animals. The presence of subliminal visual perception might be suspected in cattle because of their low level of perceptual discrimination ability of visual cues relative to humans (e.g. Phillips and Weiguo, 1991), despite their sensory apparatus being similar in many respects. Experiments were therefore conducted to determine the extent of cattle colour perception and examine whether the effects of colour on cattle behaviour transcend that their perceptual abilities. We sought to a) confirm that cattle are dichromats, taking account of stimulus brightness, which has not always been the case in previous experiments investigating cattle colour vision, and b) investigate whether cattle exhibit differences in behaviour in isoluminant primary colours for trichromatic vision. Confirmation that cattle are dichromats, together with demonstrations of differences in behaviour in the three primary colours would suggest the existence of subliminal perception, and would question the validity of determining animal welfare requirements solely on psychophysical testing of supraliminal perception.