The ability to arrange brightly fluorescent nanoscale materials into well-defined patterns is critically important in advanced optoelectronic structures. Traditional methods for doing so generally involve depositing different color quantum dot "inks," irradiating reactive (e.g., cross-linkable) ligands at their surface, and then lifting off the unexposed sections in a developer solvent. Here, we outline a fundamentally different approach for directly patterning the emission color of nanocomposite thin films utilizing mask-based lithographic techniques and laser scanning methods. In this system, a polymer film containing cesium lead halide nanocrystals (NCs) is embedded with an organohalide─termed a "photohalide generator"─which undergoes a light-triggered, perovskite-catalyzed reduction and release of halide anion for uptake by the NC lattice, markedly shifting its band gap. In this manner, a blue emitting (CsPbBr