Optical properties of plasmonic ITO nanocrystal gels, assembled by thermoreversible cobalt terpyridine links, were tuned systematically based on the size and doping concentration of the nanocrystals and length of the custom ligand molecules. Correlation of optical shifts upon assembly with nanocrystal spacing deduced by small angle X-ray scattering was used to develop a universal structure-property relationship that was validated by large-scale optical simulations on gels made using Brownian dynamics simulations.