TFA describes a nice distinction. A value like lightens exists as a ramp of intensity, bound at each end by zero and one. Hue, on the other hand, is effectively

TFA describes a nice distinction. A value like lightens exists as a ramp of intensity, bound at each end by zero and one. Hue, on the other hand, is effectively a range of differences, each blending to the other. I can think of few other phenomena which are measured in this way. Personality type is the only one which comes to mind.