There is reasonable agreement between the orientation of the major axes of the a*b* tolerance ellipses in the blue region and the slope of the tritanopic confusion lines passing through their centers. This agreement includes some experimental facts such as the approximate constant orientation of the ellipses when the luminance of the center changes and the clockwise rotation of the major axis of the ellipses when the luminous source D65 is replaced by A. However, the tilts predicted by the tritanopic lines do not follow the pattern proposed by the CIEDE2000 color-difference fortmula; in particular, for the blue-green centers with hue angles lower than 255degrees, the high tilts predicted by the tritanopic lines are quite different. From 132 ellipses previously reported, we found that the orientations of the 17 ellipses in the blue region (255degrees < h < 320degrees) were similarly predicted by both CIEDE2000 and the tritanopic confusion lines (average error of 12.2degrees in absolute value).