The model is simple: For flicker luminance stimuli and maximum purity, the hue cycle's relative luminance (computed from CIE data) is reciprocal to relative saturation. Brightness/luminance ratio B/L is proportional to relative saturation S, i.e., B/L = 1.5 S¹/⁴. S times B/L ratio gives relative saturation for brightness stimuli; just as relative luminance times B/L ratio gives brightness. Predictions for any purity agree with data on saturation discrimination, color appearance in CIE space, B/L, and CIE brightness Vb. Predictions support Hunt's concept of “colorfulness” and indicate its causal role in proporitionality of S and B/L.