Thermally activated delayed fluorescence (TADF) has been considered an important development in organic light-emitting diodes (OLEDs) for significantly enhancing efficiency of pure organic emitters. However, TADF is often associated with boarder emission spectra not meeting the requirements of modern high-performance flat-panel displays. A recent breakthrough in TADF emitters is the development of multiple resonance (MR) emitters which have a narrow spectral band width, i.e., good colour purities. However, so far molecular design for MR emitters is still much restricted and their emission peaks are covering only in a very limited range between ~ 460 to 510 nm. Herein, by exploiting a new emitting mechanism of densely packed dimer enhanced MR TADF, we demonstrate for the first time of highly efficient electroluminescence covering the RGB full colour with narrow spectral widths using pure organic emitters. MR-structured compounds with symmetry-forbidden n-π* transition for fluorescence are employed. They form intimate molecular interaction in their dimer states, leading to substantial changes in the S1 electronic structure into π-π* transition and much smaller singlet-triplet energy offset, which can significantly enhance TADF characteristics. The fluorescence efficiency increases tremendously to approach unity upon dimer formation. More importantly, molecular relaxations are strongly restricted in the systems due to their robust MR typed monomer frameworks as well as their strong dimer interaction. By applying these MR dimers in OLEDs, highly efficient narrow emission spectra can be achieved with full-width at half maximum of 32, 44, and 64 nm for blue, green, and red, respectively. Particularly, the green OLED realizes a remarkable maximum external quantum efficiency of 31%. Our strategies not only provide a pathway for realizing narrow emission covering full RGB emission range via intermolecular emitting systems (dimers, excimers, exciplexes, etc.) for the first time, but also exploit a new emitting mechanism leading to state-of-the-art performance among all reported OLEDs.