Nonvolatile, soft photonic films that reflect UV/vis light were prepared by enthalpy-driven swelling of lamellar-forming polystyrene-b-poly(2-vinylpyridine) (PS-P2VP) block copolymer thin films with a neat protic solvent. These films are very sensitive to further swelling with the addition of a small amount of acid. Transmission electron microscopy and ultrasmall-angle X-ray scattering of the films before and after the addition of the neat protic solvent revealed selective swelling of the P2VP phase while maintaining the lamellar morphology due to the presence of the layered glassy PS domains. P2VP is swollen due to the hydrogen bonding between a hydroxy group of a protic solvent and the pyridyl group of P2VP. The interdomain distance of the neat PS-P2VP film as measured by U-SAXS increased by about 200% and the films acted as a 1D photonic crystal reflecting UV light. Moreover, by exposing the neat PS-P2VP films to a mixture of a protic solvent and a small amount of sulfonic acid that can protonate the pyridyl groups of the P2VP block, the degree of swelling, therefore the interdomain distance and the wavelength of the reflection light, became significantly larger, resulting in color variations across the visible spectrum and suggesting that such a nonvolatile material system can be a sensor of the acid concentration in the millimolar regime for anhydrous solutions.