What computation does the human brain perform when we experience 'red', 'green', 'yellow', or 'blue'? How does the human visual system combine the retinal cone signals (L, M, S cones) to yield these colour sensations? To address this question we ask observers to select the colour which appears 'neither red nor green' (yielding unique yellow and unique blue) or 'neither yellow nor blue' (yielding unique red and unique green). Since we obtain these unique hue settings at different luminance and saturation levels, we have numerous points in 3-dimensional (LMS) cone space that correspond to a particular hue. This allows us to determine quantitatively how the cone signals are related to a particular perceived hue. Our results show that the human visual system employs as least three chromatic mechanisms to yield the four unique hues and, to a first approximation, these mechanisms combine the (L,M,S) cone signals linearly. Furthermore, the variability between observers is relatively small when expressed in terms of preceptual errors. Our results add further weight to the idea that the colour vision system in adult human is able to recalibrate itself based on the prior visual experience and that our colour appearance mechanisms may be a consequence of environmental contraints.