Flexible, highly conductive, robust nitrogen (N), sulfur (S) co-doped graphitic hollow carbon fibers (CFs) were directly fabricated by vacuum carbonization of "human hairs". By engineering these cost-effective CFs with polydiacetylene (PDA), a new family of ultrasensitive, reversible electrochromatic fibers that display switchable color transition (blue ⇋ red) upon exposure to current and UV-light stimuli was successfully created.