In the given excerpt, the technique of constructing a melody from changing timbres is called "klangfarbenmelodie." Klangfarbenmelodie is a German term that means "tone color melody," and it refers to a technique used in music composition where a melody is created by changing the tone color or timbre of a single note or a small group of notes. This technique is often associated with the expressionist movement in music during the early 20th century and was used by composers such as Arnold Schoenberg and Anton Webern.