Tuning the emission color of polymer light-emitting devices (PLEDs) based on a conducting polymer poly(9,9-dioctylfluorene) (PDOF) through the maskless dye-diffusion technique has been attempted. Introduction of green-emitting dye Coumarin 6 into PDOF through this technique results in green emission from the PLEDs without notable alteration of the external quantum efficiency, although the onset voltage for emission has much increased. This result suggests that Coumarin 6 plays a role of carrier trap in the polymer.