Communication: Color timing of the electro- and photoluminescence from naphthalenevinylene-PPV-based light-emitting diodes between yellow-green and red is reported. Band structure considerations are employed to explain that chemical selectivity and selective band-gap tuning are observed in some of the copolymers studied but not in others. It is also found that light emission can be tuned by changing the thickness of the copolymer film in the LED.