We have successfully showed the color mixing between fluorescence and phosphorescence by the incomplete Forster energy transfer. The emission color from fluorescent Rubrene and phosphorescent Ir(ppy)3 was found possible to be mixed for a single-layer device doped with both dyes, maintaining the high luminance efficiency of the phosphorescence. We also found out that the Forster energy transfer was very efficient and independent of the applied voltage with no loss of energy during the transfer.