As described in Methods, knowledge of the radial distribution function of the colloids, together with expressions for the isolated pair extinction cross-sections with perpendicular and parallel polarizations, can be used to construct a pairwise approximation for the ensemble extinction cross-section. This approximation, based on a sum of independent pairwise coupling contributions, becomes exact at low particle volume fraction, but loses accuracy as nanoparticle concentration increases due to neglect of many-body mutual polarization effects.