We present polymer-attenuated Coulombic self-assembly, a versatile strategy that uses neutral polymers to precisely tune the overlap of electrical double layers, enabling oppositely charged colloids in water to form a range of binary crystal structures selected by size ratio and stabilized for manipulation outside the solvent. Using an index-matched, fluorescent colloidal platform, we directly image crystallization in three dimensions with single-particle resolution, identify structures via simulated scattering, and track defect motion, melting, and twinning in real time. These experiments reveal a non-classical pathway in which amorphous precursors evolve into binary nanocrystals that grow by monomer addition, cluster capture, and oriented attachment. Continuous dialysis allows us to modulate interaction strength during growth, yielding diverse morphologies, including previously unreported hollow and composite heteroepitaxial crystals. This combination of tunable assembly and direct visualization offers a powerful model system for testing and refining theories of charged soft matter.