Structural Colors from Ultrathin Lignin Films

Abstract Structural colors offer a sustainable alternative to toxic pigments and dyes, yet their realization from lignin, the most abundant aromatic biopolymer, has remained elusive. Whereas cellulose has enabled a wide range of bio‐based photonic architectures, lignin has primarily been explored in the form of colloidal nanoparticles. Here, a paradigm shift is demonstrated: vivid, tunable structural colors generated directly from ultrathin lignin films. By simply varying film thickness, a continuous color palette spanning the visible spectrum, overcoming the need for nanoparticle synthesis or self‐assembly, is accessed. The films show high stiffness, modest angular dependence, and rapid humidity responsiveness, pointing toward scalable applications in coatings, sensors, and sustainable photonics. This work redefines lignin from an industrial waste product to a functional optical material, opening new directions in biomass‐based coloration.