Subjective colour spaces were reconstructed for persons occupationally exposed to mercury (Hg) and patients with diabetes mellitus type 2 (DM-2), two groups at risk for acquired colourvision deficiency, and compared with healthy normal trichromats. Judgments of colour dissimilarity were collected with the method of triads, applied to a composite assortment of colour samples. These were drawn from two widely-used colour arrangement tests -ten hues from the Farnsworth D-15 test and five from the Lanthony Desaturated D-15d test, ensuring that the assortment sampled two levels of lightness and saturation. The data were analysed with Maximum-Likelihood multidimensional scaling (MDS) and within a novel IndividualDifferences MDS model to estimate subject-specific parameters. The MDS solutions for the two clinical groups showed a compression along a Blue-Yellow axis, limited however to desaturated hues. This result was confirmed by the individual-differences model. In addition, the clinical groups were found to place significantly higher weights on the lightness differences between stimuli, conceivably to compensate for their reduced chromatic discrimination. The