Our aim was to characterise the higher-order chromatic mechanisms that yield the four unique hues: red, green, yellow and blue. Our results are consistent with the hypothesis that all four unique hues are generated by mechanisms that are linear in XYZ space. Furthermore, the variability between observers is relatively small when expressed in terms of perceptual errors and, as a consequence, the same linear model (same weights) fits the data for all observers. Given the linearity of these higher-order colour appearance mechanisms and the consistency across observers, these appearance judgments may be suitable to derive a standard observer model for colour appearance. Our results add further weight to the idea that the colour vision system in adult humans is able to recalibrate itself based on prior visual experience.