Pd nanoparticles are used across a broad range of applications, and thus techniques that enable their high-throughput, label-free detection, as well as in situ reaction monitoring at catalytically relevant sizes, are needed to establish robust structure-function relationships. Optical microscopy is a widespread strategy for performing these studies, but direct detection of single Pd nanoparticle scattering is limited to particles >50 nm in size due to their high damping and low cross-section. Here, we show that wavelength-resolved interferometric scattering (iSCAT) overcomes this size limitation, enabling high-throughput imaging and spectroscopy of individual Pd nanoparticles over a range of sizes and applications. For Pd octahedra with ~130 nm tip-to-tip distance, we spectroscopically discriminate synthetic targets from structural outliers based on their scattering resonances and spectral lineshapes, using simulations to validate the structure-spectral relationships. We then demonstrate direct detection of single Pd nanocubes with ~ 20 nm edge length, which lack scattering resonances but can be visualized with high signal-to-noise. By monitoring their dissolution, we reveal inter-particle heterogeneity and demonstrate our sensitivity to Pd nanocubes even smaller than ~20 nm, a size regime relevant for catalysis and nanoparticle growth. Furthermore, we monitor the reaction of individual Pd cubes with HAuCl4 at different illumination wavelengths, uncovering both heterogeneity in reaction kinetics as well as wavelength-dependent, light-driven chemistry. The results highlight the power of wavelength-resolved iSCAT for studying Pd nanoparticle transformations in situ, in real time, and without the need for extrinsic labels, revealing hidden heterogeneity and enabling new strategies for understanding Pd reactivity at the nanoscale.