Building on a framework from electrostatics, here we establish a mutual polarization method (MPM) capable of rapidly simulating the frequency-dependent optical response of large, complex assemblies of spherical nanoparticles with arbitrary dielectric functions (Fig. ), accommodating configurations with particle overlaps and triply periodic boundary conditions. We use this approach to predict and interpret experimental spectra for disordered systems with characteristic features that comprise more nanoparticles than would be practical to analyze by conventional electromagnetic simulations: 1) two-dimensional randomly mixed binary superlattices and 2) three-dimensional nanoparticle gel networks. Our analysis shows how far-field spectral features emerge from heterogeneous, structure-dependent near-field coupling and illustrates how MPM helps integrate data from structural and optical measurements to provide insights about ordering across length scales in complex assemblies. . Coupling redshifts and broadens the LSPR peaks for silver and ITO at high concentration (configuration shown in inset).