In this paper, a color difference is defined instead of the conventional color differences to evaluate gamut-mapping techniques for electronic endoscope images reproduced on CRT under environmental illuminant. This color difference is defined from Mahalanobis distance by using covariance matrices for differences of metric lightness, chroma and hue angle. The covariance matrices for endoscope images were obtained by psychophysical experiments. We compared the resultant matrices with those for natural scenes, and found the color difference was customized for the electronic endoscope images well.