I was kind of surprised to discover that there really wasn't a single source that collected all of the Funktionen pieces. By far, the best place to start was a series released in the 1990s under the series name "Acousmatrix", one volume of which was dedicated to Koenig's electronic music. I picked it up, and discovered that although it was a fine representation of Koenig's work, it didn't contain a complete set of the Funktion pieces [the 2cd set included Funktion Rot, Funktion Grau, Funktion Violett, Funktion Blau, and Funktion Indigo, but not the "green" and "yellow" and "orange" pieces].
More recently, the Dutch label BVHAAST has reissued the entire Acousmatrix series, both as a 9 disc box, and individually. The Koenig release is BVHAAST 9001-02. More on this below.
Then, I ran across a Sub Rosa compilation "Institute of Sonology 1959-1969, Early Electronic Music" [Sub Rosa SR 164], which contained "Funktion Orange." So far, so good.
The package that showed up on my door yesterday afternoon completes the series. The recent Edition RZ release of Gottfied Michael Koenig's work includes both the missing Funktion Grun and Funktion Gelb pieces, in addition to some other early WDR studio releases I wasn't familiar with and a good sampling of his software-generated compositions in the hands of human musicians. In some ways, this might be the more interesting collection, since it highlights the
connections between his compositional practice in the context of both the "tape music" and "instrumental music" worlds.
If this is the kind of thing that interests you, the easiest place to find it is probably here, through Forced Exposure.