The development of ultra-high-definition (UHD) displays demands organic light-emitting diodes (OLEDs) with high color purity of all three primary colors for a wide color gamut and high brightness essential for future AR/VR applications. However, the vibronic coupling in organic emitters typically results in broad emissions, with a full width at half maximum (FWHM) exceeding 40-50 nm. Herein, multicolor organic single-crystal microcavity light-emitting diodes (SC-MC-OLEDs) are demonstrated by embedding ultrathin 2D organic single crystals (2D-OSCs) between two silver layers that serve as both electrodes and mirrors. By leveraging the microcavity effect, the resonant output frequencies of SC-MC-OLEDs can be continuously tuned from 448 to 602 nm by adjusting the thickness of 2D-OSCs (i.e., the microcavity length), achieving high color purity with a full width at half maximum (FWHM) of <
10 nm. Furthermore, the Purcell effect in SC-MC-OLEDs enhances the radiative rate and improves light-coupling efficiency, resulting in a maximum external quantum efficiency (EQE) of up to 4% and minimal efficiency roll-off. Due to the excellent bipolar transport properties of OSCs, the brightness of SC-MC-OLEDs surpasses 10