Optically selective and thermally insulating (OSTI) covers, such as polyethylene aerogels (PEAs), have recently been proposed to improve subambient daytime radiative cooling performance by minimizing parasitic solar absorption and heat gain at the emitter. We investigate the addition of zinc sulfide (ZnS) nanoparticles inside PEAs to improve their optical selectivity. Solving for multiple scattering effects using the radiative transfer equation and Mie theory, we model the optical properties of PEA covers with different ZnS concentrations and particle diameters. Our theoretical and experimental results show that ZnS particles inside PEAs can significantly reduce solar transmittance (<0.01) while maintaining high infrared transmittance (>0.8). Our modeling also demonstrates that ZnS-pigmented PEA covers enable higher subambient cooling powers (up to 53.8  W  /  m² higher) and lower stagnation temperatures than conventional PEA under direct solar radiation (1000  W  /  m²). Finally, we investigate spatial distribution of ZnS within the cover and show that confining the ZnS near the air–cover boundary can reduce the total ZnS mass required by 59% or can enable 4% higher cooling power. Our findings promise to improve the performance of subambient radiative coolers and enable their application in passive cooling of buildings and refrigeration of food produce.