The fabrication of molecular structures with a desired morphology, e.g., nanotubes, nanoribbons, nanosprings, and sponges, is essential for the advancement of nanotechnology. Unfortunately, realization of this objective is expensive and complicated. Here, we report that irradiating a film comprising azobenzene derivatives with UV light produces oriented arrays of helical nanofilaments via the photoisomerization-induced Weigert effect. As a result, structural colors are observed due to the extrinsic chiral reflection in the visible wavelength range, and the reflected color can be tuned by adjusting the molecular length of the azobenzene derivative. This simple fabrication method can be used for fabricating large, reversible, and patternable color reflectors, providing a new platform for interference-based structural coloration as it exists in nature, such as morpho butterflies, green-winged teal, and various beetles.