After 500 hours exposure in the Xenotest 150 of samples of the European and American blue standards and visual and colorimetric measurement of the fading, a series of conclusions are published. These include comments that (1) there is no fixed relationship between visual and colorimetric evaluation of contrast; (2) there is no fixed relationship between the European and L scales; (3) the visual spacing of the European scale is regular up to standard 7, but this not the case for the American standards; and (4) the colorimetric spacing of the European standards is irregular, but that of the American standards is regular.