(a) Experimental and simulated reflection spectra at stepwise increasing ambient humidity from 10 to 90% RH. (b) The simulated electric-field distribution for MHM nanocavity with the initial H_PVA of 164 nm at 10% RH and fully-swelled H_PVA of 295 nm at 90% RH. (c) CIE 1931 diagram of the real-time color change of MHM nanocavities between 10% and 90% RH. H_PVA values for (a) and (c) are selected as 147, 164 and 169 nm, respectively.