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In recent years, perovskite quantum dots (PQDs) with the incorporation of transition-metal ions for reducing toxicity and enhancing stability have been in a research hotspot. However, the PQDs containing transition-metal ions are quite unstable and display low photoluminescence quantum yields (PLQYs), owing to the existence of defects. Herein, we have endeavored to introduce simultaneous cation and anion exchange with cobalt and chloride ions into the host CH3NH3Pb1–x Co x Br3–2x Cl2x PQDs followed by matrix encapsulation via poly­(methyl methacrylate) (PMMA) as a favorable approach to stabilize PQDs with improved PLQY. We have utilized cobalt chloride as the precursor for doping ions. The emission wavelength can be regulated from green to cyan color with the increase in doping concentration without negotiating on color quality, which is believed to be the modification of band gap and corresponding electron density estimated from the density functional theory. A high PLQY of 95% is observed even after the incorporation of cobalt and chloride ions into host PQDs. We demonstrate white light-emitting diode (LED) prototypes for backlight applications using a PQDs/PMMA composite on a blue LED chip, which exhibits excellent stability toward an ambient and robust atmosphere and provides a wide-color gamut of 124% of the NTSC standard. This research paves a way for the research community to synthesize highly efficient, color-tunable PQDs for wide-color backlighting applications.