Monodisperse, highly negatively charged, cross-linked polystyrene nanoparticles with diameters between 80 and 120 nm have been incorporated into a polyacrylamide hydrogel, where they display an iridescent color that conventionally is attributed to the so-called photonic crystal effect. The film is of red color if placed in plain water but turns to green in the presence of a 1 mM solution of an electrolyte such as sodium chloride and to purple in 100 mM solutions of electrolytes. Quantitative reflection spectroscopy was performed at various wavelengths and resulted in plots of reflected light wavelength versus ionic strength (IS) that are almost linear in the logarithmic concentration range from 5 × 10(-5) to 10(-2) mol·L(-1). We show that such films are capable of monitoring the IS of aqueous solutions in the pH range from 5 to 9. We also show that, in addition to visual and instrumental readout, the sensor films can be analyzed with a digital camera at fixed angle. The digital images were separated into their red, green, and blue channels and analyzed. The red channel was found to be best suited for determination of the IS and resulted in calibration plots that are comparable if not better than those obtained by reflectometry.