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Our aim was to characterise the chromatic mechanisms that yield the four unique hues: red, green, yellow and blue. We measured the null planes for all four unique hues and report the following two main results. (1) We confirm that three chromatic mechanisms are required to account for the four unique hues. These three chromatic mechanisms do not coincide with the chromatic tuning found in parvocellular LGN neurones, i.e., neurones tuned to L−M and S−(L+M); these subcortical chromatic mechanisms are hence not the neural substrate of the perceptual unique hues and further higher-order colour mechanisms need to be postulated. Our results are consistent with the idea that the two higher-order colour mechanisms that yield unique red and unique green respectively combine the incremental and decremental responses of the subcortical chromatic mechanisms with different weights. In contrast, unique yellow …