Intraobserver and interobserver variability in perceptual,small color difference evaluation are known but are rarely analyzed phenomena. They raise the question if humans are equipped to make reliable and uniform estimates of this kind. Data from two recent well-controlled studies show that intraobserver and, particularly, interobserver variability are surprisingly large. The use of a gray reference scale in place of a single gray reference pair has a small normative effect on the estimate. A large interobserver variability in the estimated magnitude of chromatic differences compared with achromatic reference pairs remains, with an average ratio between largest and smallest estimate of over 3:1 and ratios for individual observers and sample pairs ranging rip to 28:1. No correlation was found between the reliability of judgment and the judged magnitude of difference: highly reliable observer groups are not more consistent. Relative reliability of estimate was found inversely related to magnitude of perceived difference. Consequences of these results for the development of improved color difference formulas are discussed. (C) 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 367-374, 2009; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/col.20522