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Available colour‐discrimination data for surface colours have been analysed to produce chromaticity‐discrimination ellipses for individual colour centres. Applying simulated errors showed that when the distribution of samples around a standard was unsatisfactory, the ellipses obtained were unreliable. A total of 132 reliable ellipses were obtained. The values of 6 and a/b varied systematically over the chro‐maticity diagram, the patterns from acceptability, perceptibility, textile, and non‐textile ellipses being very similar. New experimental data involving over 400 pairs of samples corresponding to 70 of the colour centres and assessed against a grey scale were used to adjust the relative sizes of the ellipses. Overall set factors were used to bring data from different sources onto a common scale. However, even after allowing for different lightness levels, the sizes of the ellipses still appeared to be irregular. Using the new results to adjust the sizes of the individual ellipses from any one group of data, e.g., BFD, DF, or MMB, gave ellipses forming a much more regular pattern. In contrast to the MacAdam ellipses, the smallest ellipses occurred in the blue‐grey region. Our results imply that the relative sizes of the ellipses from the same group of data are in error by a factor of two or more. Comparing colour differences for different colours appears to be much more difficult than has been previously realised. The problem appears to have affected all the major groups of published data.

The JPC79 colour–difference formula has represented a substantial improvement over earlier formulae and is being applied successfully in industrial shade passing. Modifications are described which overcome certain problems and, to some extent, simplify the formula. With available experimental data, the modified version performed even better than the original formula. Perceptibility data are fitted better by increasing the JPC79 lightness weighting by a factor of two. The new formula, designated CMC (: c), has the best overall performance of any formula so far published.

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The available experimental data relating to small colour differences between pairs of surface colours have been combined together into sets including perceptibility results and acceptability results. Supplementary experiments have been carried out to enable all the previous visual results to be brought on to a common scale, and to provide extra information when this was considered necessary. A new colour‐difference formula, BFD(l:c), has been developed using the combined experimental results. Various aspects of colour differences have been considered in turn to decide the form of formula required, and the constants in the formula have been optimised using the combined perceptibility and acceptability results. The new formula is similar in structure to the CMC(l:c) formula in most respects. However, it was found that a new term was required to take account of the fact that when chromaticity discrimination ellipses calculated from experimental results are plotted in a b space, they do not all point towards the neutral point. The experimental results were not very consistent with respect to possible tilting of discrimination ellipsoids relative to the xy plane. Overall it seems that any such tilting is quite small and in the direction implicit in the CMC and BFD formulae. Experimental results based on both acceptability and perceptibility judgements form part of the same overall pattern except for the weighting of lightness differences relative to hue and chroma differences.

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We studied the individual variability of asymmetric metameric colour matching between computer displays and object colour stimuli in conditions typical for the surface colour industries. Using two different computational techniques, we assessed the contribution of observer metamerism to this variability. In the studied conditions of spatially separated computer display and surface colour stimuli, this contribution was found to be insignificant for all colours but neutrals. In the chromaticness plane, the range of matches made by different observers practically coincides with the range of matches made by an individual observer. Consequently, we conclude that in the task of matching spatially separated display and surface colours, the range of matches made by a group of observers cannot be determined from variations in their colour-matching functions, and thus the paradigm of the Standard Deviate Observer is shown to be inapplicable to the studied conditions. We suggest that individual variability in these conditions is governed by mechanisms of chromatic discrimination, and can be modeled by advanced colour difference formulae with suitably adjusted parametric coefficients.