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Traditional Fabry–Perot (F–P) structures comprising "metal/interference medium/phase-change material (PCM)" (MIP) nanostructures have achieved a color gamut close to the standard RGB (sRGB) space. However, the narrow absorption band in the visible region hinders further improvement in the gamut. Nonetheless, a large gamut is critical for display and imaging devices. This paper proposes a reflector nanostructure consisting of "metal/interference medium/phase-change materials (PCMs)/interference medium" (MIPI) to regulate the optical reflection in the visible waveband. We performed simulations to demonstrate that the addition of a 100–250-nm-thick interference medium causes the structure to change from a single to compound interference effect, thus significantly expanding the gamut (51% larger than that of the traditional MIP structure). The influences of the structure geometry and phase changes of the PCM layer on the spectral regulation were analyzed by combining experimental and simulation methods. The results indicate that the proposed structure covers almost the entire sRGB gamut. Moreover, adding the second interference medium induces a longer optical path of the incident light, which renders the structure color-sensitive to the observation angle, thus allowing for the demonstration of an anti-counterfeiting application. This nanoscale MIPI design can serve as a basis for the realization of color-regulation devices and encryption applications. [ABSTRACT FROM AUTHOR]