The study reports a conceptual model for spatiotemporal control of upconversion dynamics and photochromic evolution of Er³⁺ through interfacial energy transfer (IET) in a core-shell nanostructure. The design involves a Yb sublattice sensitization interlayer, which enhances the absorption of excitation energy and improves upconversion. The spatial manipulation of interfacial interactions between Er and Yb sublattices further contributes to upconversion. The red/green color-switchable upconversion of Er³⁺ is achieved through temporal modulation techniques such as non-steady-state excitation and time-gating. This work provides versatile designs and dynamic management of emission colors from luminescent materials, offering potential applications in optical anti-counterfeiting and speed monitoring. The results highlight the importance of IET in enhancing upconversion and tuning emission colors, with implications for smart luminescent materials and advanced photonic applications.The study reports a conceptual model for spatiotemporal control of upconversion dynamics and photochromic evolution of Er³⁺ through interfacial energy transfer (IET) in a core-shell nanostructure. The design involves a Yb sublattice sensitization interlayer, which enhances the absorption of excitation energy and improves upconversion. The spatial manipulation of interfacial interactions between Er and Yb sublattices further contributes to upconversion. The red/green color-switchable upconversion of Er³⁺ is achieved through temporal modulation techniques such as non-steady-state excitation and time-gating. This work provides versatile designs and dynamic management of emission colors from luminescent materials, offering potential applications in optical anti-counterfeiting and speed monitoring. The results highlight the importance of IET in enhancing upconversion and tuning emission colors, with implications for smart luminescent materials and advanced photonic applications.