Fano‐Engineered Au@Se Core–Shell Colloidal Pigments for Dual‐Mode Structural Color

Colloidal structural colorants are attractive because they are substrate‐free and solution‐processable, but single‐component plasmonic or dielectric colloids usually exhibit broad visible resonances that limit chroma. Here we report Au@Se core–shell colloidal pigments in which a plasmonic Au core is embedded within a visible‐range Mie‐resonant Se shell. The key signature is not the gradual red shift expected from passive dielectric loading, but an asymmetric hybrid feature with compressed linewidth, consistent with Fano‐like interference between the broad electric‐dipole response of the Se shell and the narrower Au‐core electric‐dipole response. The hybrid mode yields Q factors up to ≈15 in freely dispersed colloidal pigments, producing narrow transmission windows and spectrally separated reflection bands from the same liquid dispersion. By independently tuning the Au core diameter and Se shell thickness in a benchtop synthesis, we achieve broad visible color tuning in transmission and reflection and enlarge the accessible international commission on illumination chromaticity (CIE) area relative to single‐component Au or Se colloids. The particles are formulated into ultraviolet (UV)‐curable resins and writing inks for 254‐ppi color filters, freeform paper deposition, and polarization‐selective readout. This work establishes a colloidal design rule in which the shell acts as an active visible resonator rather than a passive refractive‐index coating.