Dynamic low-saturated structural colors empowered by a metasurface with symmetry-protected quasi-bound states in the continuum.

Low-saturation structural colors, such as muted or pastel hues, offer visual comfort, reduced eye fatigue, and aesthetic versatility for human-centric optical applications yet remain largely unexplored compared to their vivid counterparts. Here, we numerically investigate a thermally tunable metasurface that generates such colors by leveraging symmetry-protected bound states in the continuum (SP-BICs). Introducing controlled asymmetry into a dual-period silicon grating on a metallic substrate activates high-Q quasi-BIC resonances, selectively absorbing narrow spectral bands while maintaining >90% reflectance elsewhere. This results in structural colors with saturation below 10% in the CIE 1931 space. Integrated joule heating enables dynamic tuning of the absorption peak (~0.1 nm/K), allowing continuous control of pastel hues from 460 to 520 nm. As a simulation-based proof of concept, this design provides a promising route toward eye-friendly displays, low-glare optical labeling, and thermal sensing where low-saturation coloration is desired.