Two physical models, the replacing model and the filling model, have been proposed to explain the mechanism of the effective-refractive-index change of photonic crystals (PCs). Theoretically, the photonic bandgap (PBG) would shift when PCs are exposed to different environments, which leads to a change of the effective refractive index of PCs. However, the mechanism of effective-refractive-index change is distinct when PCs are placed in different gases or vapors. Here, silica PCs were successfully fabricated by a self-assembly method. The PBG of silica PCs exhibit similar redshifts in volatile organic compounds (VOCs) and in small-molecule gases. We propose a replacing model to elaborate silica PCs exposed to VOCs and a filling model to explain silica PCs exposed to small-molecule gases. Additionally, the redshift of PBG was transformed into the volume fraction of adsorbed gas in order to achieve selective sensing of silica PCs. The approach could be exploited for applications in sensing.