Subjective colour spaces were reconstructed for persons occupationally exposed to mercury (Hg) and patients with diabetes mellitus type 2, two groups at risk for acquired colour-vision 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—10 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 analyzed with maximum-likelihood multidimensional scaling (MDS) and within a novel individual-differences 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 specific form of colour-space distortion in the clinical groups indicated an increase in their thresholds for blue-yellow signals, providing insights into the nature of impairment mechanisms. The results have implications for stimuli and diagnostic procedures for testing individual differences in color vision, and for analyzing the responses. This approach is sensitive to distinctive patterns of subtle colour-vision impairment underestimated by the conventional D-15d test
