Color is primarily Quality. Secondly, it is also Weight, for it has not only color value but also brilliance. Thirdly, it is Measure, for besides Quality and Weight, it has its limits, its area, and its extent, all of which may be measured. Tone value is primarily Weight, but in its extent and its boundaries, it is also Measure. Line, however, is solely Measure.

"On Modern Art," lecture, Kunstverein, Jena (26 January 1924), trans. Paul Findlay in Paul Klee: On Modern Art (London, 1948)