There are a number of problems for which colour-vectors need to be ordered: vector morphology and colour-to-greyscale conversion are two important examples. A lexicographic ordering can perform this function, but while the luminance and saturation components have clearly defined scales with fixed origins there is no such scale for the hue dimension. In this paper we tackle this problem directly using a greyscale assignment paradigm. Observers were shown pairs of iso-luminant and iso-saturation colours, and asked to assign greyscale values to them such that the assigned greyscale difference matched their perceived colour difference. From the resulting magnitudes we construct a 1D scale of hue values using multi-dimensional scaling, and the results show a sinusoid-like ordering. We also analyse the signs of the differences (i.e. which colour was assigned a brighter greylevel), and find that different observers reliably use the same sign-assignment strategy. This result can be used to fix the phase of the sinusoidal scale resulting from the magnitude experiments. This result also supplements earlier work on the Helmholtz-Kohlrausch effect, but is not the same as we have uniquely used colors that were set as iso-luminant and iso-saturation individually for each observer. Here we visualise the effect by encoding iso-luminant and iso-saturation images with the derived greyscale ordering.