AbstractA hypothesis is proposed to predict the perceived lightness of chromatic object colors, which is well known as the Helmholtz‐Kohlrausch effect or the lightness‐reflectance ratio effect (L/Yeffect). The hypothesis is, “When two chromatic object colors with different hues have the same values for each of three attributes (whiteness, blackness, and chromaticness), the two colors have the same perceived lightness, including a chromatic‐component contribution.” The hypothesis is tested by two kinds of experiments: perceivedlightness matching using the NCS Color Atlas, and chromatic tiles determined by Wyszecki. Both results strongly confirm the hypothesis. Further, contour lines of constantL/Yratios are estimated, and are very similar to those reported p