(Chapter 18) Footnote 54:

The color of the coccyx is no different from that of any other bone that has been prepared by boiling. But later in this chapter (see note 68 below) Vesalius comments on its reddish color before preparation. He cautiously (ac veluti colore) admits here the notion of color on the strength of a dubious etymology relating ko/kkuc, “cuckoo,” to ko/kkoj, “scarlet” from the color of the berry or gall of the kermes oak, which was used to produce a scarlet dye.