Full-color synthesis was achieved, for what we believe is the first time, utilizing a novel 3x1 hard polymer-clad fiber coupler along with red, green, and blue (RGB) LED primaries. By using RGB LEDs that are coupled to three input ports, the device rendered full color from the output port with a circular emitting pixel of 135 microm in diameter with an extended color gamut. The proposed fiber-optic color synthesizer can provide a compact waveguide solution for the beam scanning display and the tunable pure white source for LED backlighting.