Binary blends of the acceptor conjugated polymer poly(2,2'-(3,3'-dioctyl-2,2'-bithienylene)-6,6'-bis(4-phenylenequinoline)) (POBTPQ) with donor conjugated polymer poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) or poly(3-octylthiophene) (POT) were nanophase-separated and observed to exhibit efficient electroluminescence in light-emitting-diodes. The 90-120 nm phase-separated morphology of MEH-PPV-containing blends was characteristic of dimixing by spinodal decomposition whereas that of POT-containing blends consisted of nucleation and growth type spherical domains (50-400 nm) dispersed in a matrix. Efficient Forster energy transfer was observed in both blend systems. Voltage-tunable orange-red <----> yellow <----> green electroluminescence was observed in the POBTPQ MEH-PPV blend diodes at a composition of 10-80 wt % MEH-PPV. Only red emission characteristic of POT was observed from POBTPQ:POT blend devices due to efficient energy transfer from POBTPQ. Large enhancements in performance of the bipolar blend light-emitting diodes were observed compared to that of the homopolymer diodes. The compositional dependence of luminance and external quantum efficiency of the blend devices was very different for the MEH-PPV and POT blends, reflecting the difference in morphology. Electric-field-induced photoluminescence quenching confirmed the bipolar charge transport in the blends and associated improved electron-hole recombination and device efficiencies. These results demonstrate that efficient electroluminescence can be achieved in bipolar blends of conjugated polymers and that nanopbase-separated morphology is essential to voltage-tunable multicolor light emission in blend devices.