Only after students grasped the intricacies of lines and planes—and could find these forms in nature—did Klee introduce colour. His theories primarily drew upon Johann Wolfgang von Goethe’s color wheel, put forth a century earlier, in 1809, which proposed the idea that red opposed green, orange opposed blue, and yellow opposed violet.
Klee added a new dimension to this diagram, turning it into a sphere, with white at the top and black at the base. This framework, he taught, should encompass all aspects of color, including hue, saturation, and value. Klee required his students to create colour diagrams of their own, including one assignment in which they visually weighed one colour against another—the color red, as it turns out, is heavier than the color blue.
While grounded in science, Klee was also a romantic when it came to colour. He often made connections between colour and music, explaining that combinations of colours (much like musical notes) can be harmonious or dissonant depending on the pairing. He would sometimes even play the violin for his students.