AbstractThe pigmentation plan used for production of the color cards made available by the Optical Society of America (OSA) for its Committee on Uniform Color Scales (UCS) was designed in such a manner that the color scales should, within the production tolerances, appear uniform in all phases of daylight. The production specifications were based onD65of the Commission International de L'Eclairage (CIE) and the CIE supplementary observer (1964) for 10° visual‐field subtense. To test for the intended invariance of uniformity of the scales in daylight, for normal observers, the effects on color differences between all nearest neighbors of the OSA colors have been studied for CIE Illuminant C with the 1931 observer, and for a “daylight” fluorescent luminaire (color temperature 6500 K) with both the 1931 and 1964 CIE observers. Although the colorimetric specifications (Y, x, y) of each color card are different for those three illuminant + observer combinations, the color differences computed with the formula of the OSA‐UCS committee are, within the production tolerances, unchanged. The purpose of this article is to show how well the aim and expectation is fulfilled—that the uniformity of color differences between nearest neighbors in the scales of the OSA‐UCS colors be essentially unchanged for normal observers and for ordinary variations of the quality of natural and artificial daylight. This invariance is found, even for daylight‐quality fluoresc