We present a light-responsive strategy to reversibly modulate Coulombic interactions between colloidal particles by leveraging a photoacid that alters particle surface charge upon illumination. This control arises from the reversible physisorption of photoacid dissociation products onto colloid surfaces, enabling dynamic tuning of interparticle forces without altering particle composition or bulk solution chemistry. Using this approach, we achieve spatially and temporally resolved crystallization via both depletion and electrostatic interactions. Real-time confocal microscopy reveals both classical and non-classical crystallization pathways, while coarse-grained molecular dynamics simulations closely recapitulate the observed behavior, providing mechanistic insight. Finally, we demonstrate programmable control over crystal formation and morphology—including localized assembly, light-guided crystal sculpting, and cyclic annealing—to fabricate larger, more ordered colloidal structures.