We report here the utilization of a ternary polymer blend system consisting of two conjugated polymers, the orange emitting poly(2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and blue emitting poly(9,9-dioctylfluorene) (PFO), with the inert matrix polymer poly(methyl methacrylate) (PMMA) to show voltage-controlled color tuning in micro- and nano-phase-separated domains. Morphological characterization using atomic force microscopy showed 100-600 nm diameter sized conjugated polymer domains depressed into the surface when processed from toluene. Fluorescence microscopy and transmission electron microscopy were utilized to show that the conjugated polymers, present as the minority phase relative to PMMA, tend to remain together in domains phase separated from the matrix. Photophysical and electroluminescence studies showed efficient Forster energy transfer from PFO to MEH-PPV when equal concentrations of conjugated polymers were utilized with emission occurring at 570 nm, leading to a bright yellow light-emitting device. Ternary blends containing an excess of PFO relative to MEH-PPV showed voltage tunable (8-12 V) yellow to green electroluminescence. The ternary blend devices showed higher efficiencies than the binary blend devices consisting of PFO/PMMA. These results demonstrate that the phase-separated morphology having an excess of the high band gap polymer is essential for obtaining voltage-controlled variable color emission in polymer light-emitting devices.