The author belabours his point that teaching music (math) in the manner described is bad.
The author tortures the reader with a whole new story, now discussing painting as analogous to math.
In the same manner that standard math education was compared to learning music without sound, the author now uses the idea of learning painting without paint.
By describing the learning of techniques that are not painting but are merely preparatory, and doing of paint-by-numbers, the author implies that students don’t get to do any actual math until rather late in their education, and even then it has no room for creative composition. The author is trying to lead the reader to accept the idea of math as an art and that teaching it as something else is contrary to reason.
Having belaboured his point with two metaphors (music and painting), the author begins to actually write was he means more plainly. He criticizes math education for not nurturing students’ desire to make patterns, and being practically all wrong in what is done.