A tunable Fabry–Pérot resonator is realized using metal–insulator–metal structure, in which the insulator is chitosan hydrogel. The chitosan swells in response to changes in relative humidity; this change affects transmissive structural color of the multilayer structure. This tunable resonator is utilized for a humidity sensor combined with a photovoltaic cell. The change in current through the photovoltaic cell provides rapid precise measurement of relative humidity, and the change in color of the multilayer provides an approximate, remotely‐readable estimate. The response requires no power, so the device has numerous sensing applications. Tunable interferometric colors are realized by silver–chitosan–silver Fabry–Pérot resonators. The chitosan is stimuli‐responsive to external vapor, so it is swollen and deswollen in response to humidity condition. Therefore, the peak of transmission shifts as humidity changes. As proof of concept, a real‐time colorimetric humidity sensor combined with a photovoltaic cell is demonstrated.