Color appearance can be specified by a procedure of direct hue scaling. In this procedure, subjects look at a stimulus and then simply state the proportions of their sensations using the four unique hue names red, yellow, green, and blue; to completeness, they also state the apparent saturation. Observers can scale stimuli quickly and reliably, and this is true even if they are relatively inexperienced. Thus stimuli can be rescaled whenever viewing conditions change such that a new specification of appearance is required. The scaled sensory values elicited by a set of stimuli are used to derive the locations of the stimuli on a color diagram that is based on appearance and which we term a Uniform Appearance Diagram (UAD). The orthogonal axes of these space are red-green and yellow-blue; the location of a stimulus specifies its hue and its distance from the origin specifies its apparent saturation. We have investigated the uniformity of this space by using a subject's UAD, for a particular set of viewing conditions, to predict both small and large color differences under comparable viewing conditions. For small-scale differences we compared wavelength discrimination functions derived from UADs with those obtained by direct adjustment of a bipartite field. For large-scale differences, subjects rated the degree of similarity of pairs of different wavelengths; these ratings were compared with the distances separating the same pairs of wavelengths on a UAD. In both cases, the agreements were very good, implying that UAD's are metrically uniform. Thus, UADs could be used to adjust the hues in a pseudo-color display so that all transitions would be equally perceptible or would differ by specified amounts.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.