The purpose of this study was to analyze whether quantitative evaluation of the color of thetongue substance using L*a*b* color coordinates system could minimize the problems arising fromthe different illuminating conditions or not. In controlled 4 different illuminating conditions (bynatural light, flashlight, f-number, shutter speed), 12 healthy subjects were photographed of theirtongue substance through a digital camera (C-2100uz, Olympus Co.), both on the top surface andon the bottom surface of the tongue substance by two examiners, twice at 3 day intervals.Clinician evaluation was also performed grading the redness of the tongue substance in the formof 5-points scale by 6 clinicians. As a result, there was no significant difference in color differencesbetween the color of the tongue substance and the reference red card in the 4 differentilluminating conditions. Intra-rater reliability was satisfied and even though limitedly, inter-raterreliability was satisfied. Color differences were significantly correlated with the results by theclinicians, although they were applicable limitedly to specific illuminating conditions. Our resultsindicate that the application of the color differences in tongue diagnosis could not only evaluatethe color information quantitatively, but also minimize the problems arising from the differentilluminating conditions and that there was the significant difference in the visual evaluation ofthe red color of the tongue substance, both between the clinicians and between the illuminatingconditions.