"Farb" is a word used in war reenactments. It means a person who participates in the reenactment without proper dedication to the craft, or someone who refuses to fully engage in playing the part.

"Farb" was the worst insult in the hardcore vocabulary. It referred to reenactors who approached the past with a lack of verisimilitude. The word's etymology was obscure; Young guessed that "farb" was short for "far-be-it-from-authentic," or possibly a respelling of "barf." Violations serious enough to earn the slur included wearing a wristwatch, smoking cigarettes, smearing oneself with sunblock or insect repellent--or, worst of all, fake blood. Farb was also a fungible word; it could become an adjective (farby), a verb (as in, "don't farb out on me"), an adverb (farbily) and a heretical school of thought (Farbism or Farbiness).
(Horwitz, Confederates in the Attic, Google Books)

The use of the word "farb" in Confederates in the Attic shows the author's commitment to learning about and immersing in the reenactment culture. To those who commit to reenactments (or "living historians," as their preferred description), "farbing" shows lack of respect for history, for the battles and the people in them, for one's fellows, and for the viewer, who expects to see a professional, accurate presentation.