The relative contribution the primary visual cortex and extrastriate visual areas make to instantaneous colour constancy (ICC) in human vision was investigated using a number of visual psychophysical techniques and also fMRI. We wanted to establish whether the conscious perception of surround colour changes caused by changes of illuminant in the same eye is a necessary condition for the normal functioning of ICC mechanisms. The results reveal no statistically significant difference in ICC between monocular and binocular conditions. The ICC index is however much reduced when the test patch and the Mondrian surround are viewed dichoptically. The results also show that the perceived colour of the surround Mondrian patches in the test eye can be strongly influenced by the presence of a similar Mondrian in the conditioning eye. Surprisingly, the strength of ICC achieved in such dichoptic colour fusing experiments is completely independent of “perceived” changes of illuminant colour in the Mondrian surround and depends only on the monocular changes of surround illuminant. The fMRI studies compared levels of cortical activation elicited when viewing coloured Mondrians under a number of different conditions. Patch luminances and chromaticities were varied independently, either randomly or as dictated by changes of illuminant. The results show that spatial changes in chromatic context, either as a result of material or illuminant changes cause significant activation in the primary visual cortex and less so in other extra-striate areas. These findings point to the neural substrates in V1 that are rich in monocularly driven neurons, as the principal locus for instantaneous colour constancy in human vision.