High performance synthetic fibers are increasingly applied in fields where they might be exposed to elevated temperatures. Above a critical temperature, Tc, however, they lose their mechanical properties resulting in unexpected failure under load. To enable a passive sensor monitoring the temperature history, we introduce a method to functionalize polymer fibers with ultrathin thermochromic optical coatings by Magnetron sputtering based on chalcogenide phase change materials. The optical contrast for the amorphous-to-crystalline transition of GeSbTe (GST) upon heating to Tc was enhanced by reflectors. We found, that the color change can be adjusted by the Ge content to the demanded temperature range matching Tc of industrially important polymer fibers.