In five experiments we measured the amplitude of the haemodynamic response to visual patterns using near infrared spectroscopy of the visual cortex. The patterns were gratings with bars that differed in chromaticity but not in luminance. In all experiments, with a wide range of chromaticities of the grating bars, the amplitude of the haemodynamic response increased with the separation of the chromaticities in the CIE 1976 UCS diagram. The amplitude did not vary consistently with the cone activation, or with the signal in colour difference channels. In four further experiments, again with a wide range of chromaticities, the gratings were rated for visual comfort. Discomfort increased consistently with the separation of the chromaticities. Given that a large haemodynamic response to patterns is generally associated with headache, we suggest that the discomfort may be a homeostatic signal to reduce sustained metabolic load on the visual cortex. © 2013.