What I've found so far is that the usual colors were Black, Green, Blue or Gold. However, no one says what they stood for. One source said that the color of the laces sent a message as well, but says nothing about what that message might be.
I've read "Voluptuous Panic" and the biography of Anita Berber by Mel Gordon, as well as Ian Buruma's essay in "Glitter and Doom."
I've checked wikipedia, I've googled "Weimar Berlin Boot Girl," "Boot code," "Boot Girls color code."
Does anyone know of a source that actually details the meaning of the color code?
ETA: Just checked JSTOR per a suggestion. Still not having much luck.
ETA 2: I just received an email from the author of Voluptuous Panic. He gave me two primary sources, which should prove entertaining to track down, and a couple of photos, but did not explain in detail. Thomas von Rheine, MASSAGE-INSTITUTE  (vol 1) and
STIEFELMÄDCHEN  (vol 2).