According to
http://www.cybozone.com/luthier/instruments/purfling.html
Purfling is used for more than aesthetics; it helps reinforce the edge grain of the thin pieces used to construct a musical instrument; it prevents splitting and checking. Most purflings are made from thin veneers of diffused porous hardwoods such as holly, or boxwood. The veneers are dyed with aniline stains or vegetable dyes to yield a rainbow of colors, selected to compliment the ornamentation of the instrument. Often times instead of simple strips, purfling can be a mosaic design that matches or contrasts with the marquetries used in the rosette

But what would I know? I only went to the web site to be sure I knew what the word meant!