All-inorganic cesium lead halide based perovskite nanocrystals (PNCs) exhibit promising optoelectronic properties, but their poor stability and anion exchange reaction limit their broad commercial applications. Herein, we demonstrated the successful synthesis of blue-green-red emitting CsPbX₃ (X = Cl/Br, Br, and Br/I) PNCs via hot injection method, followed by silica-coating and embedding in poly(methylmethacrylate) (PMMA) matrix. The photoluminescence (PL) spectra of SiO₂/PMMA-coated PNCs can be tuned continuously by regulating precursor composition ratio, from blue (CsPb(Cl_0.5/Br_0.5)₃; 460 nm) to red (CsPb(Br_0.4/I_0.6)₃ via green (CsPbBr₃; 519 nm). The PNCs composite films exhibit improved stability (thermal-, moisture-, and photo-stability) because of the barrier formed by SiO₂/PMMA coating and also displayed exceptional photoluminescent quantum yield (PLQY of blue, green, and red-emitting SiO₂/PMMA coated PNCs are 37%, 86%, and 71%, respectively) with longer lifetimes inhibiting anion exchange. Eventually, the PNCs-encapsulated SiO₂/PMMA composite films were integrated into the UV LED chip as down-converting materials to construct a prototype white-peLED unit. The designed white-peLED unit demonstrated bright white light generating CIE coordinates (0.349, 0.350), a luminous efficiency (LE) of 39.2% and a color rendering index (CRI) of 84.7. The wide color gamut of 121.47% of NTSC and 98.56% of Rec. 2020 is also achieved with the built w-LED system. Therefore, the results demonstrated that CsPbX₃ (X = Cl/Br, Br, and Br/I) PNCs@SiO₂/PMMA composite films can be employed as efficient UV to visible color conversion materials for white-LEDs and backlighting.