Three-dimensional (3D) air-gap metal-coated nanocavities with tunable geometries, changeable heights, and improved smoothness are fabricated by combining electron beam lithography (EBL), ultra dilute hydrofluoric acid solution wet etching (UDHFE), and metal magnetron sputtering technologies. With different shapes, heights, and separations of the nanocavities, the strong electromagnetic resonances inside the nanocavities are changed in different extent, resulting in broad gamut and sophisticated plasmonic color generation. The nanocavities-based metasurface is also used to construct a real-time and label-free refractive index sensor with 372 nm/RIU sensitivity, which shows distinct colorimetric change between different mediums. This nanocavities may find extensive potential applications in high-fidelity color printing, high-density information storage, and on-chip colorimetric label-free biomedical sensing.