a, Measured reflectance spectra for electron-beam-patterned PEDOT:PSS-based optical cavities in the swollen state. The colour palette from Fig. 4b is included as an inset, with certain applied doses labelled. The spectra for different stripes of the palette are plotted and linked to their applied dose. Increasing dose, which corresponds to lower swelling, blueshifts the reflectance minima. b, Detailed list of doses for strips that show different colours, along with the relative height of the cavities compared with the 4,000 µC cm−2 strip. The exposure was performed using a 50-kV acceleration voltage with an initial dry film thickness of roughly 90 nm. c, The resonance wavelength (λ min ), determined from the reflectance minimum of each cavity (a), is plotted against relative cavity height (h). Resonances are achieved when the round-trip phase shift in the cavity is an integer multiple of 2π, or ϕ total = 2πm, in which m is the integer resonance order. The total phase shift combines the propagation phase from a round trip (ϕ propagation ∝ h/λ) and the reflection phase from the mirrors (ϕ reflections ). When the cavity height is sufficiently large, higher-order resonances begin to appear. The lines of best fit for the data points attributed to the first and second resonant orders yield slopes of 3.28 (R2 = 0.999) and 0.98 (R2 = 0.996), respectively. d, The chromaticity coordinates of different strips, determined from their measured spectra, plotted in the CIE 1931 colour space. Many hues, as represented by the angular component of chromaticity, are accessible.