Biopolymers, such as alginate and chitosan, can be coated into thin films that exhibit structural color. This type of interference-based coloration depends on thickness and refractive index of the polymer. We have made use of the noted ability of biopolymers to bind metal ions, in combination with the structural coloration afforded by the thin film structure, as a basis for color-based optical sensing of metal ions in aqueous solutions. Crosslinked films of alginate, chitosan and a number of thiol-modified chitosans have been examined. Changes in film thickness, refractive index and reflectivity in response to metal ions have been measured. A number of these films possess sensitivity to Cr(III) and Cr(VI) in solution, while others exhibit a muted response to chromium in favor of other cations of interest, such as Hg(II) and Ni(II). By combining a number of different films in a multiplex sensor, detection of a variety of dissolved metallic contaminants is possible.