This study presents a method for spatiotemporal control of photochromic upconversion in erbium (Er³⁺) ions through interfacial energy transfer (IET) in a core-shell-shell nanostructure. The design involves a Yb sublattice sensitization interlayer, which enhances the absorption of excitation energy and improves upconversion efficiency. The red/green color-switchable upconversion of Er³⁺ is achieved through temporal modulation of non-steady-state excitation and time-gating techniques. The core-shell-shell nanostructure consists of a luminescent NaErF₄:Ho (0.5 mol%) core, a NaYbF₄ sensitization interlayer, and an inert NaYF₄ protective shell. The presence of the NaYbF₄ interlayer enhances the upconversion emissions of Er³⁺ via Yb³⁺-to-Er³⁺ IET. The model enables precise control of upconversion emission colors by manipulating interfacial interactions between Er and Yb sublattices. The results demonstrate that the upconversion quantum yield is increased, and the emission colors can be tuned by adjusting pump laser power, pulse width, and temporal observation window. The study also shows that the upconversion emission can be spatially controlled by inserting a thin NaYF₄ interlayer between the core and sensitization layer, which reduces energy loss due to backward energy transfer. Temporal control of upconversion is achieved by modulating the pulse width of the 980 nm excitation laser, enabling color-switchable emissions. The results have potential applications in optical anti-counterfeiting, speed monitoring, and other photonic applications. The study highlights the importance of interfacial energy transfer in controlling upconversion dynamics and provides a conceptual model for the dynamic management of emission colors in luminescent materials.This study presents a method for spatiotemporal control of photochromic upconversion in erbium (Er³⁺) ions through interfacial energy transfer (IET) in a core-shell-shell nanostructure. The design involves a Yb sublattice sensitization interlayer, which enhances the absorption of excitation energy and improves upconversion efficiency. The red/green color-switchable upconversion of Er³⁺ is achieved through temporal modulation of non-steady-state excitation and time-gating techniques. The core-shell-shell nanostructure consists of a luminescent NaErF₄:Ho (0.5 mol%) core, a NaYbF₄ sensitization interlayer, and an inert NaYF₄ protective shell. The presence of the NaYbF₄ interlayer enhances the upconversion emissions of Er³⁺ via Yb³⁺-to-Er³⁺ IET. The model enables precise control of upconversion emission colors by manipulating interfacial interactions between Er and Yb sublattices. The results demonstrate that the upconversion quantum yield is increased, and the emission colors can be tuned by adjusting pump laser power, pulse width, and temporal observation window. The study also shows that the upconversion emission can be spatially controlled by inserting a thin NaYF₄ interlayer between the core and sensitization layer, which reduces energy loss due to backward energy transfer. Temporal control of upconversion is achieved by modulating the pulse width of the 980 nm excitation laser, enabling color-switchable emissions. The results have potential applications in optical anti-counterfeiting, speed monitoring, and other photonic applications. The study highlights the importance of interfacial energy transfer in controlling upconversion dynamics and provides a conceptual model for the dynamic management of emission colors in luminescent materials.